[X86] Add the vector integer min/max instructions to isAssociativeAndCommutative.

As far as I know these should be freely reassociatable just like
the floating point MAXC/MINC instructions.

The *reduce* test changes are largely regressions and caused by
the "generic" CPU we default to not having a scheduler model.

The machine-combiner-int-vec.ll test shows the positive benefits
of this change.

Differential Revision: https://reviews.llvm.org/D62787

llvm-svn: 362629