Revert "Allow merging bundles that have a fixed-reg def"

[elliottt]

Reverts bytecodealliance/regalloc2#155

Fuzzing discovered a case that caused the panic on line 1241 of src/ion/process.rs to trigger.

[Amanieu]

Can you share a reproducer for this?

[jameysharp]

Here's a base64-encoded fuzz input I ran into today (after about a half-million iterations of the ion_checker fuzz target), separate from the oss-fuzz case that Trevor is responding to:
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The debug representation of that fuzz input is:

    TestCase {
        func: {
          REF: v0
          REF: v1
          REF: v2
          REF: v3
          REF: v4
          REF: v5
          REF: v6
          REF: v7
          REF: v8
          REF: v9
          REF: v10
          REF: v11
          REF: v12
          REF: v13
          REF: v14
          REF: v15
          REF: v16
          REF: v17
          REF: v18
          REF: v19
          REF: v20
          REF: v21
          REF: v22
          REF: v23
          REF: v24
          REF: v25
          REF: v26
          REF: v27
          REF: v28
          REF: v29
          REF: v30
          REF: v31
          REF: v32
          REF: v33
          REF: v34
          REF: v35
          block0(): # succs:[1] preds:[]
            inst0: Op ops:[Def@Early: v0f reg] clobber:[]
            inst1: Op ops:[Def: v1f reuse(1), Use: v0f reg] clobber:[]
            inst2: Op ops:[Def: v2f reuse(1), Use: v1f reg] clobber:[]
            inst3: Op ops:[Def: v3f reuse(1), Use: v1f reg] clobber:[]
            inst4: Op ops:[Def@Early: v4f reg, Use: v3f any] clobber:[]
            inst5: Branch ops:[] clobber:[]
            params: block1(v4)
          block1(v5): # succs:[2] preds:[0, 3]
            inst6: Op ops:[Def: v6f fixed(p0f), Use: v5f reg, Use: v0f fixed(p0f)] clobber:[]
            inst7: Op ops:[Def: v7f any, Fixed: p63f] clobber:[]
            inst8: Op ops:[Def: v8f reuse(1), Use: v6f reg] clobber:[]
            inst9: Op ops:[Def: v9f reuse(1), Use: v6f reg] clobber:[]
            inst10: Branch ops:[] clobber:[]
            params: block2(v1)
          block2(v10): # succs:[3, 4] preds:[1]
            inst11: Op ops:[Def: v11f reuse(1), Use: v10f reg] clobber:[]
            inst12: Op ops:[Def: v12f reuse(1), Use: v11f reg] clobber:[]
            inst13: Op ops:[Def@Early: v13i reg, Use: v11f reg] clobber:[]
            inst14: Op ops:[Def@Early: v14i reg, Use: v11f reg] clobber:[]
            inst15: Branch ops:[] clobber:[]
            params: block3(v6), block4(v6)
          block3(v15): # succs:[1] preds:[2]
            inst16: Op ops:[Def: v16f reuse(1), Use: v15f reg] clobber:[]
            inst17: Op ops:[Def: v17f reuse(1), Use: v16f reg] clobber:[]
            inst18: Op ops:[Def: v18f reuse(1), Use: v16f reg] clobber:[]
            inst19: Op ops:[Def: v19f reuse(1), Use: v16f reg] clobber:[]
            inst20: Op ops:[Def: v20f reuse(1), Use: v16f reg] clobber:[]
            inst21: Op ops:[Def: v21f reuse(1), Use: v16f reg] clobber:[]
            inst22: Op ops:[Def: v22f reg] clobber:[]
            inst23: Op ops:[Def: v23f any] clobber:[]
            inst24: Op ops:[Def: v24f any] clobber:[]
            inst25: Op ops:[Def: v25f any] clobber:[]
            inst26: Branch ops:[] clobber:[]
            params: block1(v15)
          block4(v26): # succs:[5] preds:[2]
            inst27: Op ops:[Def: v27f any] clobber:[]
            inst28: Op ops:[Def: v28f any] clobber:[]
            inst29: Op ops:[Def: v29f reuse(1), Use: v26f reg, Use: v26f any, Use: v26f any] clobber:[]
            inst30: Op ops:[Def: v30f reg] clobber:[]
            inst31: Branch ops:[] clobber:[]
            params: block5(v29)
          block5(v31): # succs:[] preds:[4]
            inst32: Op ops:[Def: v32f any, Use: v3f any, Use: v3f any] clobber:[PReg(hw = 0, class = Int, index = 0)]
            inst33: Op ops:[Def: v33f any] clobber:[]
            inst34: Op ops:[Def: v34f any, Use: v31f any, Use: v31f any] clobber:[]
            inst35: Op ops:[Def: v35f any] clobber:[]
            inst36: Ret ops:[] clobber:[]
        }
        ,
    }

[elliottt]

Thanks @jameysharp, the one from the fuzzbug was six times larger!