[ConstraintElim] Rename test file to match contents.

The test file contains tests with chained/nested GEPs. Reflect that in
the name.

diff --git a/llvm/test/Transforms/ConstraintElimination/gep-add.ll b/llvm/test/Transforms/ConstraintElimination/gep-chains.ll
similarity index 100%
rename from llvm/test/Transforms/ConstraintElimination/gep-add.ll
rename to llvm/test/Transforms/ConstraintElimination/gep-chains.ll