Don't call emit_clobber in lower-subreg.cc's resolve_simple_move.

Don't call emit_clobber in lower-subreg.cc's resolve_simple_move.

Following up on posts/reviews by Segher and Uros, there's some question
over why the middle-end's lower subreg pass emits a clobber (of a
multi-word register) into the instruction stream before emitting the
sequence of moves of the word-sized parts.  This clobber interferes
with (LRA) register allocation, preventing the multi-word pseudo to
remain in the same hard registers.  This patch eliminates this
(presumably superfluous) clobber and thereby improves register allocation.

A concrete example of the observed improvement is PR target/43644.
For the test case:
__int128 foo(__int128 x, __int128 y) { return x+y; }

on x86_64-pc-linux-gnu, gcc -O2 currently generates:

foo:	movq    %rsi, %rax
        movq    %rdi, %r8
        movq    %rax, %rdi
        movq    %rdx, %rax
        movq    %rcx, %rdx
        addq    %r8, %rax
        adcq    %rdi, %rdx
        ret

with this patch, we now generate the much improved:

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

2023-05-07  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR target/43644
	* lower-subreg.cc (resolve_simple_move): Don't emit a clobber
	immediately before moving a multi-word register by parts.

gcc/testsuite/ChangeLog
	PR target/43644
	* gcc.target/i386/pr43644.c: New test case.