[SSP] [3/3] cmpxchg and addrspacecast instructions can now

trigger stack protectors.  Fixes PR42238.

Add test coverage for llvm.memset, as proxy for all llvm.mem*
intrinsics. There are two issues here: (1) they could be lowered to a
libc call, which could be intercepted, and do Bad Stuff; (2) with a
non-constant size, they could overwrite the current stack frame.

The test was mostly written by Matt Arsenault in r363169, which was
later reverted; I tweaked what he had and added the llvm.memset part.

Differential Revision: https://reviews.llvm.org/D67845

llvm-svn: 373220