LoongArch: Implement TARGET_IRA_CHANGE_PSEUDO_ALLOCNO_CLASS hook

The hook changes the allocno class to either FP_REGS or GR_REGS depending on
the mode of the register. This results in better register allocation overall,
fewer spills and reduced codesize - particularly in SPEC2017 lbm.

gcc/ChangeLog:

	* config/loongarch/loongarch.cc
	(loongarch_ira_change_pseudo_allocno_class): New function.
	(TARGET_IRA_CHANGE_PSEUDO_ALLOCNO_CLASS): Define macro.