[InstCombine] Support vectors in simplifyIntToPtrRoundTripCast.

This code compares getPointerTypeSizeInBits and getTypeSizeInBits.
getPointerTypeSizeInBits contains a call to getScalarType while
getTypeSizeInBits does not. This makes the code incorrect for vectors.

For scalable vectors this caused a warning about a scalable TypeSize
being converted to unsigned.

Switch to DL.getTypeSizeInBits for the pointers too. This should
work since inttoptr/ptrtoint can't change the number of elements.
This was suggested by @nikic in D139911.

Fixes PR59480.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D139962