[IndVars] Support AND/OR in optimizeLoopExitWithUnknownExitCount

This patch allows optimizeLoopExitWithUnknownExitCount to deal with
branches by conditions that are not immediately ICmp's, but aggregates
of ICmp's joined by arithmetic or logical AND/OR. Each ICmp is optimized
independently.

Differential Revision: https://reviews.llvm.org/D139832
Reviewed By: nikic