[IRCE][NFCI] Refactor parseRangeCheckICmp

Simplify parseRangeCheckICmp:
- If RHS is loop-variant, swap LHS/RHS and swap predicate
- all checks are either IV >(=) const or IV <(=) RHS (maybe not const)

Patch by Aleksandr Popov!

Differential Revision: https://reviews.llvm.org/D148720