[MergedLoadStoreMotion] Don't transform across may-throw calls

[MergedLoadStoreMotion] Don't transform across may-throw calls

It is unsafe to hoist a load before a function call which may throw, the
throw might prevent a pointer dereference.

Likewise, it is unsafe to sink a store after a call which may throw.
The caller might be able to observe the difference.

This fixes PR27858.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@270828 91177308-0d34-0410-b5e6-96231b3b80d8