recursively handle right hand side of `=` statement

[aviatesk]

Otherwise we will encounter error like:

julia> using FastBroadcast

julia> @macroexpand @.. broadcast=false @view(J[idxs])=@view(J[idxs]) - inv_alpha
:($(Expr(:error, "SSAValue objects should not occur in an AST")))

This happens because currently FB does not process = statement recursively, where the right hand side of the statement can be a complex IR construct.

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (d84df11) 90.83% compared to head (1a9f86c) 90.03%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #53 +/- ## ========================================== - Coverage 90.83% 90.03% -0.81% ========================================== Files 1 1 Lines 262 271 +9 ========================================== + Hits 238 244 +6 - Misses 24 27 +3 ``` | [Files](https://app.codecov.io/gh/YingboMa/FastBroadcast.jl/pull/53?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yingbo+Ma) | Coverage Δ | | |---|---|---| | [src/FastBroadcast.jl](https://app.codecov.io/gh/YingboMa/FastBroadcast.jl/pull/53?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yingbo+Ma#diff-c3JjL0Zhc3RCcm9hZGNhc3Quamw=) | `90.03% <92.50%> (-0.81%)` | :arrow_down: |

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