[llvm-c] Accept GEP operators in some APIs

As requested in D115787, I've added a test for LLVMConstGEP2 and
LLVMConstInBoundsGEP2. However, to make this work in the echo test,
I also had to change a couple of APIs to work on GEP operators,
rather than only GEP instructions.

Differential Revision: https://reviews.llvm.org/D115858