LoongArch: Try harder using vrepli instructions to materialize const vectors

For

  a = (v4si){0xdddddddd, 0xdddddddd, 0xdddddddd, 0xdddddddd}

we just want

  vrepli.b $vr0, 0xdd

but the compiler actually produces a load:

  la.local $r14,.LC0
  vld      $vr0,$r14,0

It's because we only tried vrepli.d which wouldn't work.  Try all vrepli
instructions for const int vector materializing to fix it.

gcc/ChangeLog:

	* config/loongarch/loongarch-protos.h
	(loongarch_const_vector_vrepli): New function prototype.
	* config/loongarch/loongarch.cc (loongarch_const_vector_vrepli):
	Implement.
	(loongarch_const_insns): Call loongarch_const_vector_vrepli
	instead of loongarch_const_vector_same_int_p.
	(loongarch_split_vector_move_p): Likewise.
	(loongarch_output_move): Use loongarch_const_vector_vrepli to
	pun operend[1] into a better mode if it's a const int vector,
	and decide the suffix of [x]vrepli with the new mode.
	* config/loongarch/constraints.md (YI): Call
	loongarch_const_vector_vrepli instead of
	loongarch_const_vector_same_int_p.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/vrepli.c: New test.