[InstCombine] Make flag drop during select equiv fold more generic

Instead of unsetting flags on the instruction, attempting the
fold, and the resetting the flags if it failed, add support to
simplifyWithOpReplaced() to ignore poison-generating flags/metadata
and collect all instructions where they may need to be dropped.

This allows us to perform the fold a) with poison-generating
metadata, which was previously not handled and b) poison-generating
flags/metadata that are not on the root instruction.

Proof for the ctpop case: https://alive2.llvm.org/ce/z/3H3HFs

Fixes https://github.com/llvm/llvm-project/issues/62450.

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