AMDGPU: Stop setting attributes based on TargetOptions

Having arbitrary passes looking at the TargetOptions is pretty
messy. This was also disregarding if a function already had an
explicit attribute setting on it. opt/llc now add the attributes to
functions that don't specify the attribute. clang and lld do not call
the function to do this, which they maybe should.

This was also treating unsafe-fp-math as implying the others, and
setting the other attributes based on it. This is not done anywhere
else, and I'm not sure is correct based on the current description of
the option bit.

Effectively reverts 1d8cf2be