AMDGPU: Disable some fneg combines unless nsz

AMDGPU: Disable some fneg combines unless nsz

For -(x + y) -> (-x) + (-y), if x == -y, this would
change the result from -0.0 to 0.0. Since the fma/fmad
combine is an extension of this problem it also
applies there.

fmul should be fine, and I don't think any of the unary
operators or conversions should be a problem either.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292473 91177308-0d34-0410-b5e6-96231b3b80d8