PSA: Do not use ScratchDoubleReg or ScratchFloat32Reg; they are going away

I'm about to land a patch that will use ScratchDoubleScope and
ScratchFloat32Scope everywhere on tier-1 platforms to handle the fp scratch
registers. After that I hope to hide the names ScratchDoubleReg and
ScratchFloat32Reg so that they are not used by mistake; this will break any
patches you have in flight that use these. (I see a new use managed to
land overnight while I was sleeping; nice try...) Please start using the
scopes to handle the scratch registers. I know this is sometimes difficult
but during my rewrite I encountered zero cases where it actually was
difficult.

--lars