[X86] Reimplement r321437 using custom lowering instead of as a DAG combine.

My original implementation ran as a DAG combine post type legalization, but it turns out we don't run that DAG combine step if type legalization didn't change anything. Attempts to make the combine run before type legalization as well hit other issues.

So just do it in LowerMUL where we can catch more cases.

llvm-svn: 321496