[LVI] NFC. On the fast dest path use inverse predicate instead of inverse range result

Gathering constantins from a condition on the false path ask makeAllowedICmpRegion about inverse predicate instead of inversing the resulting range.

This change was separated from the review "[LVI] Make LVI smarter about comparisons with non-constants" (https://reviews.llvm.org/D23205#inline-198361)

llvm-svn: 278009