[InstSimplify] Make simplifyWithOpReplaced() recursive (PR63104)

[InstSimplify] Make simplifyWithOpReplaced() recursive (PR63104)

Support replacement of operands not only in the immediate
instruction, but also instructions it uses.

To the most part, this extension is straightforward, but there are
two bits worth highlighting:

First, we can now no longer assume that if the Op is a vector, the
instruction also returns a vector. If Op is a vector and the
instruction returns a scalar, we should consider it as a cross-lane
operation.

Second, for the x ^ x special case, we can no longer assume that
the operand is RepOp, as we might have a replacement higher up the
instruction chain.

There is one optimization regression, but it is in a fuzzer-generated
test case.

Fixes https://github.com/llvm/llvm-project/issues/63104.