Fix asserts in AMDGCN fmed3 folding by handling more cases of NaN

Better NaN handling for AMDGCN fmed3.

All operands are checked for NaN now. The checks
were moved before the canonicalization to provide
a better mapping from fclamp. Changed the behaviour
of fmed3(x,y,NaN) to return max(x,y) instead of
min(x,y) in light of this. Updated tests as a result
and added some new cases to cover the fix.

Patch by Alan Baker

llvm-svn: 336375