x86: PR target/103611: Splitter for DST:DI = (HI:SI<<32)|LO:SI.

A common idiom is to create a DImode value from the "concat" of two SImode
values, using "(long long)hi << 32 | (long long)lo", where the operation
may be ior, xor or plus.  On x86, with -m32, the high and low parts of
a DImode register are actually different SImode registers (typically %edx
and %eax) so ideally this idiom should reduce to two move instructions
(or optimally, just clever register allocation).

Unfortunately, GCC currently performs the IOR operation above on -m32,
and worse allocates DImode registers (split to SImode register pairs)
for both the zero extended HI and LO values.

Hence, for test1 from the new test case below:

typedef int __v4si __attribute__ ((__vector_size__ (16)));
long long test1(__v4si v) {
  unsigned int loVal = (unsigned int)v[0];
  unsigned int hiVal = (unsigned int)v[1];
  return (long long)(loVal) | ((long long)(hiVal) << 32);
}

we currently generate (with -m32 -O2 -msse4.1):

test1:	subl    $28, %esp
        pextrd  $1, %xmm0, %eax
        pmovzxdq        %xmm0, %xmm1
        movq    %xmm1, 8(%esp)
        movl    %eax, %edx
        movl    8(%esp), %eax
        orl     12(%esp), %edx
        addl    $28, %esp
        orb     $0, %ah
        ret

with this patch we now generate:

test1:	pextrd  $1, %xmm0, %edx
        movd    %xmm0, %eax
        ret

The fix is to recognize and split the idiom (hi<<32)|zext(lo) prior
to register allocation on !TARGET_64BIT, simplifying this sequence to
"highpart(dst) = hi; lowpart(dst) = lo".

The one minor complication is that sse.md's define_insn for
*vec_extractv4si_0_zext_sse4 can sometimes interfere with this
optimization.  It turns out that on !TARGET_64BIT, the zero_extend:DI
following vec_select:SI isn't free, and this insn gets split back
into multiple instructions during later passes, but too late to
be optimized away by this patch/reload.  Hence the last hunk of
this patch is to restrict *vec_extractv4si_0_zext_sse4 to TARGET_64BIT.
Checking PR target/80286, where *vec_extractv4si_0_zext_sse4 was
first added, this seems reasonable.

2021-12-18  Roger Sayle  <roger@nextmovesoftware.com>
	    Uroš Bizjak  <ubizjak@gmail.com>

gcc/ChangeLog
	PR target/103611
	* config/i386/i386.md (any_or_plus): New code iterator.
	(define_split): Split (HI<<32)|zext(LO) into piece-wise
	move instructions on !TARGET_64BIT.
	* config/i386/sse.md (*vec_extractv4si_0_zext_sse4):
	Restrict to TARGET_64BIT.

gcc/testsuite/ChangeLog
	PR target/103611
	* gcc.target/i386/pr103611-2.c: New test case.