[AliasSetTracker] Do not treat experimental_guard intrinsic as memory writing instruction

The `experimental_guard` intrinsic has memory write semantics to model the thread-exiting
logic, but does not do any actual writes to memory. Currently, `AliasSetTracker` treats it as a
normal memory write. As result, a loop-invariant load cannot be hoisted out of loop because
the guard may possibly alias with it.

This patch makes `AliasSetTracker` so that it doesn't treat guards as memory writes.

Differential Revision: https://reviews.llvm.org/D50497
Reviewed By: reames

llvm-svn: 339753