[GlobalISel] LegalizerHelper - Extract widenScalarAddoSubo method
The widenScalar implementation for signed and unsigned overflowing
operations were very similar: both are checked by truncating the result
and then re-sign/zero-extending it and checking that it matches the
Using a truncate + zero-extend for the unsigned case instead of manually
producing the AND instruction like before leads to an extra copy
instruction during legalization, but this should be harmless.
Differential Revision: https://reviews.llvm.org/D95035