[NFCI][SimplifyCFG] simplifyUnreachable(): Use poison constant to represent the result of unreachable instrs
Mimics similar change for InstCombine:
ce192ced2b901be67444c481ab5ca0d731e6d982 / 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.