[SDAG] try to avoid multiply for X*Y==0

Forking this off from D140850 -
https://alive2.llvm.org/ce/z/TgBeK_
https://alive2.llvm.org/ce/z/STVD7d

We could almost justify doing this in IR, but consideration for
"minsize" requires that we only try it in codegen -- the
transform is not reversible.

In all other cases, avoiding multiply should be a win because a
mul is more expensive than simple/parallelizable compares. AArch
even has a trick to keep instruction count even for some types.

Differential Revision: https://reviews.llvm.org/D141086