[NFC][IRCE] Check that Index is AddRec in the parseRangeCheckICmp

Next step of the preparatory refactoring for the upcoming support of
new range check form to parse.

Previous one: https://reviews.llvm.org/D154156

With this change we avoid meaningless parsing after realizing that Index
is not AddRec

Reviewed By: skatkov
Differential Revision: https://reviews.llvm.org/D154158