spirv-opt: Loop unroller ignores OpPhi instructions in the loop's body

[data.zip](https://github.com/KhronosGroup/SPIRV-Tools/files/5083970/data.zip)

spirv-opt produces an error
> error: line 36: OpPhi's incoming basic block 14[%14] is not a predecessor of 301[%301].
%302 = OpPhi %float %291 %14
>
> error: line 0: Validation failed after pass loop-unroll

when executed with `spirv-opt -o out.spv --validate-after-all --loop-unroll`.

Can be reproduced on b8de4f57.

Start with data.zip and reproduce the failure using spirv-opt with --validate-after-all --loop-unroll, as described in the issue. Investigate the loop-unroll pass's handling of OpPhi instructions and verify the fix by rerunning the command without the validation error on commit b8de4f57's reproduction.

Written by the indexing model from the issue text.