InstCombine: Move Sub->Xor rule from SimplifyDemanded to InstCombine

The rule that turns a sub to xor if the LHS is 2^n-1 and the remaining bits
are known zero, does not use the demanded bits at all: Move it to the
normal InstCombine code path.

Differential Revision: http://reviews.llvm.org/D9417

llvm-svn: 236268