[Moore] Add AssignedVarOp and canonicalization for VariableOp.

[hailongSun2000]

The canonicalization for VariableOp is aimed at merging the "easy use" of the variable and its value(continuous assignment) into a new op called assigned_variable. Don't handle blocking_assign and nonBlocking_assign due to the dominance. The "easy use" is assigning a value to a variable with the whole bit-width, in other words, exclude the bits slice, concatenation operator, etc.

[hailongSun2000]

Appendix:

/home/phoenix/work/HDLBits/test01.sv:4:10: error: Unsupported drive the same wire 'l' two times now
  assign l = 1'b0;
         ^
module {
  moore.module @Foo() {
    %l = moore.net wire : <l1>
    %l_0 = moore.assigned_variable name "l" %1 : <l1>
    %0 = moore.constant true : i1
    %1 = moore.conversion %0 : !moore.i1 -> !moore.l1
    moore.assign %l, %1 : l1
    %2 = moore.constant false : i1
    %3 = moore.conversion %2 : !moore.i1 -> !moore.l1
    moore.assign %l, %3 : l1
    moore.output
  }
}

[hailongSun2000]

I'll separate the changing of SVModule into a different PR.

[fabianschuiki]

I'm wondering if this could work as a canonicalization pattern defined on VariableOp :thinking:. The pattern could theoretically go through all users of the variable op and if there is a single continuous assignment, the assigned value could be merged into a new AssignedVariableOp. But I'm not sure how idiomatic and scalable it is to scan an entire user list in a canonicalization... @uenoku may have an opinion on that :smirk:

[uenoku]

It makes sense to implement it as canonicalization. We do the same thing for SV so compile time should be OK. However in that case please make sure lowering pass can lower them even without canonicalization.

[hailongSun2000]

Hey, @fabianschuiki @uenoku! I add the canonicalization pattern for moore.variable. But it seems can not keep the original variable, if not call replaceOp() /replaceOpWithNewOp(), the canonicalize() will not take effect.

[uenoku]

Can we remove mergeAssignments pass if we implemented it as a canonicalization?

[hailongSun2000]

I have removed the MergeAssignments pass.

[hailongSun2000]

Thanks for the code review :smiley: .

[fabianschuiki]

Very cool! 🥳