[FuncSpec] Prefer DataLayout-aware constant folding of GEPs.

As shown in D154820, the DataLayout-independent constant folding
interface is not good enough for handling GEPs. Instead we should
be using the DataLayout-aware constant folding interface. Since
there isn't a method to specifically handle GEPs we can use the
one which folds generic instruction operands.

Differential Revision: https://reviews.llvm.org/D154821