Check for overflows when calculating the offset in GetGEPCost.

Summary:
This avoids C++ UB if the GEP is weird and the calculation overflows
int64_t, and it's also observable in the cost model's results.

Such GEPs are almost surely not valid pointers, but LLVM nonetheless
generates them sometimes.

Reviewers: sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38337

llvm-svn: 314362