[DataLayout] Fix occurrences that size and range of pointers are assumed to be the same.

GEP index size can be specified in the DataLayout, introduced in D42123. However, there were still places
in which getIndexSizeInBits was used interchangeably with getPointerSizeInBits. This notably caused issues
with Instcombine's visitPtrToInt; but the unit tests was incorrect, so this remained undiscovered.

Differential Revision: https://reviews.llvm.org/D68328

Patch by Joseph Faulls!