[IR] Add range accessors for the indices of a GEP instruction.

These were noticed as missing in a code review. Add them and the boring
unit test to make sure they compile and DTRT.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@296444 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed