[SCEVExpander] Fix incorrect reuse of more poisonous instructions (PR63763)

SCEVExpander tries to reuse existing instruction with the same
SCEV expression. However, doing this replacement blindly is not
safe, because the instruction might be more poisonous.

What we were already doing is to drop poison-generating flags on
the reused instruction. But this is not the only way that more
poison can be introduced. The poison-generating flag might not
be directly on the reused instruction, or the poison contribution
might come from something like 0 * %var, which folds to 0 but can
still introduce poison.

This patch fixes the issue in a principled way, by determining which
values can contribute poison to the SCEV expression, and then
checking whether any additional values can contribute poison to the
instruction being reused. Poison-generating flags are dropped if
doing that enables reuse.

This is a pretty big hammer and does cause some regressions in
tests, but less than I would have expected. I wasn't able to come
up with a less intrusive fix that still satisfies the correctness
requirements.

Fixes https://github.com/llvm/llvm-project/issues/63763.
Fixes https://github.com/llvm/llvm-project/issues/63926.
Fixes https://github.com/llvm/llvm-project/issues/64333.
Fixes https://github.com/llvm/llvm-project/issues/63727.

Differential Revision: https://reviews.llvm.org/D158181