[NFCI][SimplifyCFG] simplifyUnreachable(): Use poison constant to represent...

[NFCI][SimplifyCFG] simplifyUnreachable(): Use poison constant to represent the result of unreachable instrs

Mimics similar change for InstCombine:
  ce192ced / D104602

All these uses are in blocks that aren't reachable from function's entry,
and said blocks are removed by SimplifyCFG itself,
so we can't really test this change.