[ADT] Add APInt/MathExtras isShiftedMask variant returning mask offset/length

[ADT] Add APInt/MathExtras isShiftedMask variant returning mask offset/length

In many cases, calls to isShiftedMask are immediately followed with checks to determine the size and position of the bitmask.

This patch adds variants of APInt::isShiftedMask, isShiftedMask_32 and isShiftedMask_64 that return these values as additional arguments.

I've updated a number of cases that were either performing seperate size/position calculations or had created their own local wrapper versions of these.

Differential Revision: https://reviews.llvm.org/D119019