[IRSim][IROutliner] Allowing GEP instructions to be outlined.

We outline what the IRSimilarityIdentifier deems GEP Instructions to be
similar.  In this case, the types must be the same, and the operand
values after the first indexing operand must be the exact same.

Tests:
Removing test/Transforms/IROutliner/illegal-gep.ll
Adding test/Transforms/IROutliner/outlining-gep.ll

Reviewers: jroelofs, paquette

Differential Revision: https://reviews.llvm.org/D87317