target-mips: Use TCG registers for the FPU.

target-mips: Use TCG registers for the FPU.

With normal FP, this doesn't have much affect on the generated code,
because most of the FP operations are not CONST/PURE, and so we spill
registers in about the same frequency as the explicit load/stores.

But with Loongson multimedia instructions, which are all integral and
whose helpers are in fact CONST+PURE, this greatly improves the code.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>