LoongArch: Fix infinite secondary reloading of FCCmode [PR113148]

The GCC internal doc says:

     X might be a pseudo-register or a 'subreg' of a pseudo-register,
     which could either be in a hard register or in memory.  Use
     'true_regnum' to find out; it will return -1 if the pseudo is in
     memory and the hard register number if it is in a register.

So "MEM_P (x)" is not enough for checking if we are reloading from/to
the memory.  This bug has caused reload pass to stall and finally ICE
complaining with "maximum number of generated reload insns per insn
achieved", since r14-6814.

Check if "true_regnum (x)" is -1 besides "MEM_P (x)" to fix the issue.

gcc/ChangeLog:

	PR target/113148
	* config/loongarch/loongarch.cc (loongarch_secondary_reload):
	Check if regno == -1 besides MEM_P (x) for reloading FCCmode
	from/to FPR to/from memory.

gcc/testsuite/ChangeLog:

	PR target/113148
	* gcc.target/loongarch/pr113148.c: New test.