[AMDGPU] Only enable mad/mac legacy f32 patterns if denormals may be flushed

Following on from D88890, this makes the newly added patterns
conditional on NoFP32Denormals. mad/mac f32 instructions always flush
denormals regardless of the MODE register setting, and I believe the
legacy variants do the same.

Differential Revision: https://reviews.llvm.org/D89123