[SimplifyCFG] Constant fold a branch implied by it's incoming edge

The most common use case is when eliminating redundant range checks in an example like the following:
c = a[i+1] + a[i];

Note that all the smarts of the transform (the implication engine) is already in ValueTracking and is tested directly through InstructionSimplify.

Differential Revision: http://reviews.llvm.org/D13040

llvm-svn: 251596