Merging r326404:

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r326404 | rnk | 2018-02-28 17:19:18 -0800 (Wed, 28 Feb 2018) | 14 lines

[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
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llvm-svn: 329855