Remove f14 to f31 from FREG and FSREG.

Remove f14 to f31 from FREG and FSREG.

This would have happened later, if f14 to f31 became regvar (like r13
to r31 are now).  I am doing it now because ncg is too slow for rules
"with FREG FREG uses FREG".  We use such rules for adf 8 and other EM
instructions that operate on 2 floats.  Like my last commit cfbc537,
this commit speeds ncg by removing choices for register allocation.