[DAGCombiner][x86] add transform/hook to decompose integer multiply into shift/add

This is an alternative to D37896. I don't see a way to decompose multiplies 
generically without a target hook to tell us when it's profitable. 

ARM and AArch64 may be able to remove some duplicate code that overlaps with 
this transform.

As a first step, we're only getting the most clear wins on the vector examples
requested in PR34474:
https://bugs.llvm.org/show_bug.cgi?id=34474

As noted in the code comment, it's likely that the x86 constraints are tighter
than necessary, but it may not always be a win to replace a pmullw/pmulld.

Differential Revision: https://reviews.llvm.org/D52195

llvm-svn: 342554