[SDAG] Expand pow2 mulo using shifts

Expand MULO with constant power of two operand into a shift. The
overflow is checked with (x << shift) >> shift == x, where the right
shift will be logical for umulo and arithmetic for smulo (with
exception for multiplications by signed_min).

Differential Revision: https://reviews.llvm.org/D59041

llvm-svn: 355937