[GlobalISel] Look through extends etc in CombinerHelper::matchConstantOp

It's possible to end up with a zext or something in the way of a G_CONSTANT,
even pre-legalization. This can happen with memsets.

e.g.

https://godbolt.org/z/Bjc8cw

To make sure we can catch these cases, use `getConstantVRegValWithLookThrough`
instead of `mi_match`.

Differential Revision: https://reviews.llvm.org/D81875