InstCombine: Optimize (1 << X) Pred CstP2 to X Pred Log2(CstP2)

We may, after other optimizations, find ourselves with IR that looks
like:

  %shl = shl i32 1, %y
  %cmp = icmp ult i32 %shl, 32

Instead, we should just compare the shift count:

  %cmp = icmp ult i32 %y, 5

llvm-svn: 185242