Add tests to reproduce pointer/index width confusion crashes

Some calls to GEPOperator::accumulateConstantOffset(APInt) passed the
pointer bitwidth as the width of the APInt, while the function asserts
that the width of its argument is equal to the index width of the GEP
pointer input. These values are almost always the same, so mixing up
which call to use doesn't usually cause issues. However, when dealing
with data layouts where these values are different, the passes tested
here can crash.

This will be fixed in D143437 .

Differential Revision: https://reviews.llvm.org/D144673