Add post-reload splitter for extendditi2 on x86_64.

Add post-reload splitter for extendditi2 on x86_64.

This is another step towards a possible solution for PR 105137.
This patch introduces a define_insn for extendditi2 that allows
DImode to TImode sign-extension to be represented in the early
RTL optimizers, before being split post-reload into the exact
same idiom as currently produced by RTL expansion.

Typically this produces the identical code, so the first new
test case:
__int128 foo(long long x) { return (__int128)x; }

continues to generate:
foo:	movq    %rdi, %rax
        cqto
        ret

The "magic" is that this representation allows combine and the
other RTL optimizers to do a better job.  Hence, the second
test case:

__int128 foo(__int128 a, long long b) {
    a += ((__int128)b) << 70;
    return a;
}

which mainline with -O2 currently generates as:

foo:	movq    %rsi, %rax
        movq    %rdx, %rcx
        movq    %rdi, %rsi
        salq    $6, %rcx
        movq    %rax, %rdi
        xorl    %eax, %eax
        movq    %rcx, %rdx
        addq    %rsi, %rax
        adcq    %rdi, %rdx
        ret

with this patch now becomes:
foo:    movl    $0, %eax
        salq    $6, %rdx
        addq    %rdi, %rax
        adcq    %rsi, %rdx
        ret

i.e. the same code for the signed and unsigned extension variants.

2023-01-01  Roger Sayle  <roger@nextmovesoftware.com>
	    Uroš Bizjak  <ubizjak@gmail.com>

gcc/ChangeLog
	* config/i386/i386.md (extendditi2): New define_insn.
	(define_split): Use DWIH mode iterator to treat new extendditi2
	identically to existing extendsidi2_1.
	(define_peephole2): Likewise.
	(define_peephole2): Likewise.
	(define_Split): Likewise.

gcc/testsuite/ChangeLog
	* gcc.target/i386/extendditi2-1.c: New test case.
	* gcc.target/i386/extendditi2-2.c: Likewise.