[llvm] [RISCV][VLOPT] Look through PHI instructions (PR #132236)

[Philip Reames via llvm-commits]

preames wrote:

> Similar to what we do for copies. We may reduce one of the PHI operands and not the other, and thats perfectly okay.

Unlike COPY, PHI allows the existence of cycles in the def/use graph.  Usually, when traversing phis we need some kind of visited set to bailout if we get trapped in a cycle.  Is there some non-obvious invariant in the code here which side steps that?

Classic test case would be something like:
```
%V = some_vector_instruction
%phi = phi [%V, %loop_header], [%phi, %loop]
```


https://github.com/llvm/llvm-project/pull/132236