InstCombine: Flip the order of two urem transforms

There are two transforms in visitUrem that conflict with each other.

*) One, if a divisor is a power of two, subtracts one from the divisor
   and turns it into a bitwise-and.
*) The other unwraps both operands if they are surrounded by zext
   instructions.

Flipping the order allows the subtraction to go beneath the sign
extension.

llvm-svn: 181668