[InstSimplify] Simplify smul.fix and smul.fix.sat

Add simplification of smul.fix and smul.fix.sat according to
  X * 0 -> 0
  X * undef -> 0
  X * (1 << scale) -> X

This includes the commuted patterns and splatted vectors.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D98299