[APInt] Add APIntOps::ScaleBitMask helper

APInt is used to describe a bit mask in a variety of value tracking and demanded bits/elts functions.

When traversing through dst/src operands, we have a number of places where these masks need to widened/narrowed to translate through bitcasts, reductions etc. to a different type.

This patch add a APIntOps::ScaleBitMask common helper, adds unit test coverage, and updates a number of cases to use the the helper instead of their own implementation.

This came up on D109065 where we currently have to add yet another implementation of the same code.

Differential Revision: https://reviews.llvm.org/D109683