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[MemCpyOpt] Don't shorten memset if destination observable through unwinding

Authored by nikic on Sat, Oct 10, 8:47 AM.

Description

[MemCpyOpt] Don't shorten memset if destination observable through unwinding

MemCpyOpt can shorten a memset if it is later partially overwritten
by a memcpy. It checks that the destination is not read in between,
but we also need to make sure that the destination cannot be observed
via unwinding.

Differential Revision: https://reviews.llvm.org/D89190