LoongArch: Avoid unnecessary zero-initialization using LSX for scalar popcount

LoongArch: Avoid unnecessary zero-initialization using LSX for scalar popcount

Now for __builtin_popcountl we are getting things like

	vrepli.b	$vr0,0
	vinsgr2vr.d	$vr0,$r4,0
	vpcnt.d	$vr0,$vr0
	vpickve2gr.du	$r4,$vr0,0
	slli.w	$r4,$r4,0
	jr  $r1

The "vrepli.b" instruction is introduced by the init-regs pass (see
PR61810 and all the issues it references).  To work it around, we can
use post-reload instead of define_expand: the "f" constraint will make
the compiler automatically move the scalar between GPR and FPR, and
reload is much later than init-regs so init-regs won't get in our way.

Now the code looks like:

	movgr2fr.d	$f0,$r4
	vpcnt.d	$vr0,$vr0
	movfr2gr.d	$r4,$f0
	jr  $r1

gcc/ChangeLog:

	* config/loongarch/loongarch.md (cntmap): Change to uppercase.
	(popcount<GPR:mode>2): Modify to a post reload split.