[ValueTracking][InstSimplify] Support min/max selects in computeConstantRange()

Add support for min/max flavor selects in computeConstantRange(),
which allows us to fold comparisons of a min/max against a constant
in InstSimplify. This was suggested by spatel as an alternative
approach to D59378. I've also added the infinite looping test from
that revision here.

Differential Revision: https://reviews.llvm.org/D59506

llvm-svn: 356415