[DAGCombiner] Add simple folds for SMULFIX/UMULFIX/SMULFIXSAT

Summary:
Add the following DAGCombiner folds for mulfix being
one of SMULFIX/UMULFIX/SMULFIXSAT:
  (mulfix x, undef, scale) -> 0
  (mulfix x, 0, scale) -> 0

Also added canonicalization of constants to RHS.

Reviewers: RKSimon, craig.topper, spatel

Reviewed By: RKSimon

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66052

llvm-svn: 369103