[FuncSpec][NFC] Improve the unittest coverage for constant folding of GEPs.

The InstCostVisitor is currently using the DataLayout-independent constant
folding interface. This is a workaround since we can't directly call
ConstantExpr::getGetElementPtr due to deprecation. This patch shows that
the constant folding interface we are using is not good enough.

Differential Revision: https://reviews.llvm.org/D154820