[NFC][NARY-REASSOCIATE] Restructure code to aviod isPotentiallyReassociatable

Currently we have to duplicate the same checks in isPotentiallyReassociatable and tryReassociate. With simple pattern like add/mul this may be not a big deal. But the situation gets much worse when I try to add support for min/max. Min/Max may be represented by several instructions and can take different forms. In order reduce complexity for upcoming min/max support we need to restructure the code a bit to avoid mentioned code duplication.

Reviewed By: mkazantsev

Differential Revision: https://reviews.llvm.org/D88286