[InstCombine] combine mul+shl separated by zext

This appears to slightly help patterns similar to what's
shown in PR42874:
https://bugs.llvm.org/show_bug.cgi?id=42874
...but not in the way requested.

That fix will require some later IR and/or backend pass to
decompose multiply/shifts into something more optimal per
target. Those transforms already exist in some basic forms,
but probably need enhancing to catch more cases.

https://rise4fun.com/Alive/Qzv2

llvm-svn: 367891