[InstCombine] Do not exercise nested max/min pattern on abs

Summary:
This is a fix for assertion failure in
`getInverseMinMaxSelectPattern` when ABS is passed in as a select pattern.

We should not be invoking the simplification rule for
ABS(MIN(~ x,y))) or ABS(MAX(~x,y)) combinations.

Added a test case which would cause an assertion failure without the patch.

Reviewers: sanjoy, majnemer

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D30051

llvm-svn: 295719