LoongArch: Optimize fixed-point and floating-point conversion operations.

LoongArch: Optimize fixed-point and floating-point conversion operations.

Before optimization, the operation of taking fixed-point numbers from memory
and then forcing type conversion needs to be loaded into fixed-point registers
before conversion. After the optimization is completed, the fixed-point value
is directly transferred to the floating-point register for type conversion.

eg:
    extern int a;
    float
    test(void)
    {
      return (float)a;
    }

Assembly code before optimization:
	pcalau12i	$r12,%got_pc_hi20(a)
	ld.d	$r12,$r12,%got_pc_lo12(a)
	ldptr.w	$r12,$r12,0
	movgr2fr.w	$f0,$r12
	ffint.s.w	$f0,$f0

Optimized assembly code:
	pcalau12i	$r12,%got_pc_hi20(a)
	ld.d	$r12,$r12,%got_pc_lo12(a)
	fld.s	$f0,$r12,0
	ffint.s.w	$f0,$f0

gcc/ChangeLog:

	* config/loongarch/loongarch.md: Allows fixed-point values to be loaded
	from memory into floating-point registers.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/float-load.c: New test.