[APInt] Allow extending and truncating to the same width

Allow zext, sext, trunc, truncUSat and truncSSat to extend or truncate
to the same bit width, which is a no-op.

Disallowing this forced clients to use workarounds like using
zextOrTrunc (even though they never wanted truncation) or zextOrSelf
(even though they did not want its strange behaviour of allowing a
*smaller* bit width, which is also treated as a no-op).

Differential Revision: https://reviews.llvm.org/D125556