[IR] Handle large element size when calculating GEP indices

This is a fix for the issue reported at
https://reviews.llvm.org/D110043#3019942:
The ElementSize is a uint64_t and as such may be larger than the
index space, or be negative in the index space. This is UB, but
shouldn't cause assertion failures.

We address this by detecting whether the size is too large and
use a zero index in that case (which is always conservatively
correct).

Differential Revision: https://reviews.llvm.org/D110437