[ValueTracking] move helpers for SelectPatterns from InstCombine to ValueTracking

Most of the folds based on SelectPatternResult belong in InstSimplify rather than
InstCombine, so the helper code should be available to other passes/analysis.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326812 91177308-0d34-0410-b5e6-96231b3b80d8