[LVI] Make LVI smarter about comparisons with non-constants

Make LVI smarter about comparisons with a non-constant. For example, a s< b constraints a to be in [INT_MIN, INT_MAX) range. This is a part of https://llvm.org/bugs/show_bug.cgi?id=28620 fix.

Reviewed By: sanjoy

Differential Revision: https://reviews.llvm.org/D23205

llvm-svn: 278122