[AArch64] Unify the integer min/max vector selection patterns with the intrinsic ones

Summary:
This change lowers the aarch64 integer vector min/max intrinsic nodes to
generic min/max nodes and replaces the intrinsic selection patterns with
the generic ones.

There should already be testing in place for this, so no further tests
were added.

Reviewers: jmolloy

Subscribers: aemerson, llvm-commits, rengolin

Differential Revision: http://reviews.llvm.org/D12276

llvm-svn: 246030