ARM64: fix SELECT_CC lowering in absence of NaNs.

We were swapping the true & false results while testing for FMAX/FMIN,
but not putting them back to the original state if the later checks
failed.

Should fix PR19700.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208469 91177308-0d34-0410-b5e6-96231b3b80d8