AMDGPU: Split denormal mode tracking bits

Prepare to accurately track the future denormal-fp-math attribute
changes. The way to actually set these separately is not wired in yet.

This is just a mechanical change, and mostly still assumes the input
and output mode match. This should be refined for some cases. For
example, fcanonicalize lowering should use the flushing variant if
either input or output flushing is enabled