Merging r339411:

Merging r339411:
------------------------------------------------------------------------
r339411 | gbiv | 2018-08-10 07:14:43 +0200 (Fri, 10 Aug 2018) | 17 lines

[MemorySSA] "Fix" lifetime intrinsic handling

MemorySSA currently creates MemoryAccesses for lifetime intrinsics, and
sometimes treats them as clobbers. This may/may not be the best way
forward, but while we're doing it, we should consider
MayAlias/PartialAlias to be clobbers.

The ideal fix here is probably to remove all of this reasoning about
lifetimes from MemorySSA + put it into the passes that need to care. But
that's a wayyy broader fix that needs some consensus, and we have
miscompiles + a release branch today, and this should solve the
miscompiles just as well.

differential revision is D43269. Landing without an explicit LGTM (and
without using the special please-autoclose-this syntax) so we can still
use that revision as a place to decide what the right fix here is.

------------------------------------------------------------------------

git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_70@339545 91177308-0d34-0410-b5e6-96231b3b80d8