[GlobalOpt] prevent crashing on large integer types (PR42932)

This is a minimal fix (copy the predicate for the assert) to
prevent the crashing seen in:
https://bugs.llvm.org/show_bug.cgi?id=42932
...when converting a constant integer of arbitrary width to uint64_t.

Differential Revision: https://reviews.llvm.org/D65970

llvm-svn: 368437