LoongArch: Fixed the problem of incorrect judgment of the immediate field of...

LoongArch: Fixed the problem of incorrect judgment of the immediate field of the [x]vld/[x]vst instruction.

The [x]vld/[x]vst directive is defined as follows:
  [x]vld/[x]vst {x/v}d, rj, si12

When not modified, the immediate field of [x]vld/[x]vst is between 10 and
14 bits depending on the type. However, in loongarch_valid_offset_p, the
immediate field is restricted first, so there is no error. However, in
some cases redundant instructions will be generated, see test cases.
Now modify it according to the description in the instruction manual.

gcc/ChangeLog:

	* config/loongarch/lasx.md (lasx_mxld_<lasxfmt_f>):
	Modify the method of determining the memory offset of [x]vld/[x]vst.
	(lasx_mxst_<lasxfmt_f>): Likewise.
	* config/loongarch/loongarch.cc (loongarch_valid_offset_p): Delete.
	(loongarch_address_insns): Likewise.
	* config/loongarch/lsx.md (lsx_ld_<lsxfmt_f>): Likewise.
	(lsx_st_<lsxfmt_f>): Likewise.
	* config/loongarch/predicates.md (aq10b_operand): Likewise.
	(aq10h_operand): Likewise.
	(aq10w_operand): Likewise.
	(aq10d_operand): Likewise.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/vect-ld-st-imm12.c: New test.