[InstSimplify] Convert tests to opaque pointers (NFC)

The only interesting test change is in @PR31262, where the following
fold is now performed, while it previously was not:
https://alive2.llvm.org/ce/z/a5Qmr6

llvm/test/Transforms/InstSimplify/ConstProp/gep.ll has not been
updated, because there is a tradeoff between folding and inrange
preservation there that we may want to discuss.

Updates have been performed using:
https://gist.github.com/nikic/98357b71fd67756b0f064c9517b62a34