x86_64: Improve (interunit) moves from TImode to V1TImode.

This patch improves the code generated when moving a 128-bit value
in TImode, represented by two 64-bit registers, to V1TImode, which
is a single SSE register.

Currently, the simple move:
typedef unsigned __int128 uv1ti __attribute__ ((__vector_size__ (16)));
uv1ti foo(__int128 x) { return (uv1ti)x; }

is always transferred via memory, as:
foo:    movq    %rdi, -24(%rsp)
        movq    %rsi, -16(%rsp)
        movdqa  -24(%rsp), %xmm0
        ret

with this patch, we now generate (with -msse2):
foo:    movq    %rdi, %xmm1
        movq    %rsi, %xmm2
        punpcklqdq      %xmm2, %xmm1
        movdqa  %xmm1, %xmm0
        ret

and with -mavx2:
foo:    vmovq   %rdi, %xmm1
        vpinsrq $1, %rsi, %xmm1, %xmm0
        ret

Even more dramatic is the improvement of zero extended transfers.

uv1ti bar(unsigned char c) { return (uv1ti)(__int128)c; }

Previously generated:
bar:    movq    $0, -16(%rsp)
        movzbl  %dil, %eax
        movq    %rax, -24(%rsp)
        vmovdqa -24(%rsp), %xmm0
        ret

Now generates:
bar:    movzbl  %dil, %edi
        movq    %rdi, %xmm0
        ret

My first attempt at this functionality attempted to use a simple
define_split, but unfortunately, this triggers very late during the
compilation preventing some of the simplifications we'd like (in
combine).  For example the foo case above becomes:

foo:    movq    %rsi, -16(%rsp)
        movq    %rdi, %xmm0
        movhps  -16(%rsp), %xmm0

transferring half directly, and the other half via memory.
And for the bar case above, GCC fails to appreciate that
movq/vmovq clears the high bits, resulting in:

bar:    movzbl  %dil, %eax
        xorl    %edx, %edx
        vmovq   %rax, %xmm1
        vpinsrq $1, %rdx, %xmm1, %xmm0
        ret

Hence the solution (i.e. this patch) is to add a special case
to ix86_expand_vector_move for TImode to V1TImode transfers.

2022-01-08  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386-expand.c (ix86_expand_vector_move): Add
	special case for TImode to V1TImode moves, going via V2DImode.

gcc/testsuite/ChangeLog
	* gcc.target/i386/sse2-v1ti-mov-1.c: New test case.
	* gcc.target/i386/sse2-v1ti-zext.c: New test case.

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