[LSV] Return same bitwidth from getConstantOffset.

Previously, getConstantOffset could return an APInt with a different
bitwidth than the input pointers.  For example, we might be loading an
opaque 64-bit pointer, but stripAndAccumulateInBoundsConstantOffsets
might give a 32-bit offset.

This was OK in most cases because in gatherChains, we casted the APInt
back to the original ASPtrBits.

But it was not OK when considering selects.  We'd call getConstantOffset
twice and compare the resulting APInt's, which might not have the same
bit width.

This fixes that.  Now getConstantOffset always returns offsets with the
correct width, so we don't need the hack of casting it in gatherChains,
and it works correctly when we're handling selects.

Differential Revision: https://reviews.llvm.org/D151640