LoongArch: Optimize LSX vector shuffle on floating-point vector

LoongArch: Optimize LSX vector shuffle on floating-point vector

The vec_perm expander was wrongly defined.  GCC internal says:

Operand 3 is the “selector”.  It is an integral mode vector of the same
width and number of elements as mode M.

But we made operand 3 in the same mode as the shuffled vectors, so it
would be a FP mode vector if the shuffled vectors are FP mode.

With this mistake, the generic code manages to work around and it ends
up creating some very nasty code for a simple __builtin_shuffle (a, b,
c) where a and b are V4SF, c is V4SI:

    la.local    $r12,.LANCHOR0
    la.local    $r13,.LANCHOR1
    vld $vr1,$r12,48
    vslli.w $vr1,$vr1,2
    vld $vr2,$r12,16
    vld $vr0,$r13,0
    vld $vr3,$r13,16
    vshuf.b $vr0,$vr1,$vr1,$vr0
    vld $vr1,$r12,32
    vadd.b  $vr0,$vr0,$vr3
    vandi.b $vr0,$vr0,31
    vshuf.b $vr0,$vr1,$vr2,$vr0
    vst $vr0,$r12,0
    jr  $r1

This is obviously stupid.  Fix the expander definition and adjust
loongarch_expand_vec_perm to handle it correctly.

gcc/ChangeLog:

	* config/loongarch/lsx.md (vec_perm<mode:LSX>): Make the
	selector VIMODE.
	* config/loongarch/loongarch.cc (loongarch_expand_vec_perm):
	Use the mode of the selector (instead of the shuffled vector)
	for truncating it.  Operate on subregs in the selector mode if
	the shuffled vector has a different mode (i. e. it's a
	floating-point vector).

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/vect-shuf-fp.c: New test.