Convert an APInt to int64_t properly in TTI::getGEPCost().

Summary:
If the pointer width is 32 bits and the calculated GEP offset is
negative, we call APInt::getLimitedValue(), which does a
*zero*-extension of the offset.  That's wrong -- we should do an sext.

Fixes a bug introduced in rL314362 and found by Evgeny Astigeevich.

Reviewers: efriedma

Subscribers: sanjoy, javed.absar, llvm-commits, eastig

Differential Revision: https://reviews.llvm.org/D38557

llvm-svn: 314935