[GVN] Prevent ScalarPRE from hoisting across instructions that don't pass...

[GVN] Prevent ScalarPRE from hoisting across instructions that don't pass control flow to successors

This is to address a problem similar to those in D37460 for Scalar PRE. We should not
PRE across an instruction that may not pass execution to its successor unless it is safe
to speculatively execute it.

Differential Revision: https://reviews.llvm.org/D38619

llvm-svn: 319147