[CGP] Fix GEP issue with out of range APInt constant values not fitting in int64_t

Test case reduced from https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=7173

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339556 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed