[InstCombine] Call getCmpPredicateForMinMax only with a valid SPF

Summary:
There are `SelectPatternFlavor`s that don't represent min or max idioms,
and we should not be passing those to `getCmpPredicateForMinMax`.

Fixes PR25745.

Reviewers: majnemer

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D15249

llvm-svn: 254869