[NFC][IRCE] Extract 'IV vs Limit' parsing to a separate method

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

This change isolates logic of 'IV vs Limit' range check parsing to
simplify adding parsers for new range checks forms.

Reviewed By: skatkov
Differential Revision: https://reviews.llvm.org/D154160