[ConstantHoist] Fix APInt ctor assertion
The result here may require truncation. Fix this by removing the
calculateOffsetDiff() helper entirely. As far as I can tell, this
code does not actually have to deal with different bitwidths.
findBaseConstants() will produce ranges of constants with equal
types, which is what maximizeConstantsInRange() will then work
on.
Fixes assertion reported at:
https://github.com/llvm/llvm-project/pull/114539#issuecomment-2453008679
