PR target/94680: Clear upper bits of V2DF using movq (like V2DI).

This simple i386 patch unblocks a more significant change.  The testcase
gcc.target/i386/sse2-pr94680.c isn't quite testing what's intended, and
alas the fix for PR target/94680 doesn't (yet) handle V2DF mode.

For the first test from sse2-pr94680.c, below

v2df foo_v2df (v2df x) {
  return __builtin_shuffle (x, (v2df) { 0, 0 }, (v2di) { 0, 2 });
}

GCC on x86_64-pc-linux-gnu with -O2 currently generates:

        movhpd  .LC0(%rip), %xmm0
        ret
.LC0:
        .long   0
        .long   0

which passes the test as it contains a mov insn and no xor.
Alas reading a zero from the constant pool isn't quite the
desired implementation.  With this patch we now generate:

        movq    %xmm0, %xmm0
        ret

The same code as we generate for V2DI, and add a stricter
test case.  This implementation generalizes the sse2_movq128
to V2DI and V2DF modes using a VI8F_128 mode iterator and
renames it *sse2_movq128_<mode>.  A new define_expand is
introduced for sse2_movq128 so that the exisiting builtin
interface (CODE_FOR_sse2_movq128) remains the same.

2022-03-16  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR target/94680
	* config/i386/sse.md (sse2_movq128): New define_expand to
	preserve previous named instruction.
	(*sse2_movq128_<mode>): Renamed from sse2_movq128, and
	generalized to VI8F_128 (both V2DI and V2DF).

gcc/testsuite/ChangeLog
	PR target/94680
	* gcc.target/i386/sse2-pr94680-2.c: New stricter V2DF test case.