AArch64: Don't lower ISD::SELECT to ISD::SELECT_CC

Instead of lowering SELECT to SELECT_CC which is further lowered later
immediately call the SELECT_CC lowering code. This is preferable
because:
- Avoids an unnecessary roundtrip through the legalization queues with
  an intermediate node.
- More importantly: Lowered operations get visited last leading to SELECT_CC
  getting visited with legalized operands and unlegalized ones for preexisting
  SELECT_CC nodes. This does not hurt the current code (hence no testcase) but
  is required for another patch I am working on.

Differential Revision: http://reviews.llvm.org/D8187

llvm-svn: 234334