Fix incorrect GEP bitwidth in areNonOverlapSameBaseLoadAndStore()

When using a datalayout that has pointer index width != pointer size this
code triggers an assertion in Value::stripAndAccumulateConstantOffsets().
I encountered this this while compiling FreeBSD for CHERI-RISC-V.
Also update LoadsTest.cpp to use a DataLayout with index width != pointer
width to ensure this case is tested.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D110406