Add rotl64ti2_doubleword pattern to i386.md

Add rotl64ti2_doubleword pattern to i386.md

This patch adds rot[lr]64ti2_doubleword patterns to the x86_64 backend,
to move splitting of 128-bit TImode rotates by 64 bits after reload,
matching what we now do for 64-bit DImode rotations by 32 bits with -m32.

In theory moving when this rotation is split should have little
influence on code generation, but in practice "reload" sometimes
decides to make use of the increased flexibility to reduce the number
of registers used, and the code size, by using xchg.

For example:
__int128 x;
__int128 y;
__int128 a;
__int128 b;

void foo()
{
    unsigned __int128 t = x;
    t ^= a;
    t = (t<<64) | (t>>64);
    t ^= b;
    y = t;
}

Before:
        movq    x(%rip), %rsi
        movq    x+8(%rip), %rdi
        xorq    a(%rip), %rsi
        xorq    a+8(%rip), %rdi
        movq    %rdi, %rax
        movq    %rsi, %rdx
        xorq    b(%rip), %rax
        xorq    b+8(%rip), %rdx
        movq    %rax, y(%rip)
        movq    %rdx, y+8(%rip)
        ret

After:
        movq    x(%rip), %rax
        movq    x+8(%rip), %rdx
        xorq    a(%rip), %rax
        xorq    a+8(%rip), %rdx
        xchgq   %rdx, %rax
        xorq    b(%rip), %rax
        xorq    b+8(%rip), %rdx
        movq    %rax, y(%rip)
        movq    %rdx, y+8(%rip)
        ret

One some modern architectures this is a small win, on some older
architectures this is a small loss.  The decision which code to
generate is made in "reload", and could probably be tweaked by
register preferencing.  The much bigger win is that (eventually) all
TImode mode shifts and rotates by constants will become potential
candidates for TImode STV.

2022-07-31  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386.md (define_expand <any_rotate>ti3): For
	rotations by 64 bits use new rot[lr]64ti2_doubleword pattern.
	(rot[lr]64ti2_doubleword): New post-reload splitter.