[InstCombine] Convert a masked.load of a dereferenceable address to an unconditional load

If we have a masked.load from a location we know to be dereferenceable, we can simply issue a speculative unconditional load against that address. The key advantage is that it produces IR which is well understood by the optimizer. The select (cnd, load, passthrough) form produced should be pattern matchable back to hardware predication if profitable.

Differential Revision: https://reviews.llvm.org/D59703

llvm-svn: 359000