i386: Tweak define_insn_and_split to fix FAIL of gcc.target/i386/pr43644-2.c

This patch resolves the failure of pr43644-2.c in the testsuite, a code
quality test I added back in July, that started failing as the code GCC
generates for 128-bit values (and their parameter passing) has been in
flux.

The function:

unsigned __int128 foo(unsigned __int128 x, unsigned long long y) {
  return x+y;
}

currently generates:

foo:    movq    %rdx, %rcx
        movq    %rdi, %rax
        movq    %rsi, %rdx
        addq    %rcx, %rax
        adcq    $0, %rdx
        ret

and with this patch, we now generate:

foo:	movq    %rdi, %rax
        addq    %rdx, %rax
        movq    %rsi, %rdx
        adcq    $0, %rdx

which is optimal.

2023-12-31  Uros Bizjak  <ubizjak@gmail.com>
	    Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR target/43644
	* config/i386/i386.md (*add<dwi>3_doubleword_concat_zext): Tweak
	order of instructions after split, to minimize number of moves.

gcc/testsuite/ChangeLog
	PR target/43644
	* gcc.target/i386/pr43644-2.c: Expect 2 movq instructions.