[RelLookupTableConverter] Bail on invalid pointer size (x32)

The RelLookupTableConverter pass currently only supports 64-bit
pointers.  This is currently enforced using an isArch64Bit() check
on the target triple. However, we consider x32 to be a 64-bit target,
even though the pointers are 32-bit. (And independently of that
specific example, there may be address spaces with different pointer
sizes.)

As such, add an additional guard for the size of the pointers that
are actually part of the lookup table.

Differential Revision: https://reviews.llvm.org/D131399