AMDGPU: Redefine clamp node as clamp 0.0-1.0

Change implementation to use max instead of add.
min/max/med3 do not flush denormals regardless of the mode,
so it is OK to use it whether or not they are enabled.

Also allow using clamp with f16, and use knowledge
of dx10_clamp.

llvm-svn: 295788