R600: Permute operands when selecting legacy min/max

This gets the correct NaN behavior based on the compare type
the hardware uses. This now passes the new piglit test I have
for this on SI.

Add stricter tests for the operand order.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222079 91177308-0d34-0410-b5e6-96231b3b80d8