[APInt] Add a simpler overload of multiplicativeInverse (#87610)

The current APInt::multiplicativeInverse takes a modulus which can be
any value, but all in-tree callers use a power of two. Moreover, most
callers want to use two to the power of the width of an existing APInt,
which is awkward because 2^N is not representable as an N-bit APInt.

Add a new overload of multiplicativeInverse which implicitly uses
2^BitWidth as the modulus.