[IPSCCP] do not break musttail invariant (PR36485)
Do not replace results of `musttail` calls with a constant if the
call itself can't be removed.
Do not zap returns of `musttail` callees, if the call site can't be
removed and replaced with a constant.
Do not zap returns of `musttail`-calling blocks, this breaks
invariant too.
Patch by Fedor Indutny
Differential Revision: https://reviews.llvm.org/D43695
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326404
91177308-0d34-0410-b5e6-
96231b3b80d8
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