AMDGPU: Keep track of modifiers when converting v_mac to v_mad

Since v_max_f32_e64/v_max_f16_e64 can be folded if the target
instruction supports the clamp bit, we also need to maintain
modifiers when converting v_mac to v_mad.

This fixes a rendering issue with Dirt Rally because a v_mac
instruction with the clamp bit set was converted to a v_mad
but that bit was lost during the conversion.

Fixes: e184e01dd79 ("AMDGPU: Fold FP clamp as modifier bit")

Patch by Samuel Pitoiset <samuel.pitoiset@gmail.com>

llvm-svn: 297556