[Sink] Fix predicate in legality check

Summary:
isSafeToSpeculativelyExecute is the wrong predicate to use here.
All that checks for is whether it is safe to hoist a value due to
unaligned/un-dereferencable accesses. However, not only are we doing
sinking rather than hoisting, our concern is that the location
we're loading from may have been modified. Instead forbid sinking
any load across a critical edge.

Reviewers: majnemer

Subscribers: davide, llvm-commits

Differential Revision: https://reviews.llvm.org/D33179

llvm-svn: 305102