[InstCombine] refactor mul narrowing folds; NFCI

Similar to rL342278:
The test diffs are all cosmetic due to the change in
value naming, but I'm including that to show that the
new code does perform these folds rather than something
else in instcombine.

D52075 should be able to use this code too rather than
duplicating all of the logic.

llvm-svn: 342292