i386: Handle sign_extend like zero_extend in *concatditi3_[346]

i386: Handle sign_extend like zero_extend in *concatditi3_[346]

This patch generalizes some of the patterns in i386.md that recognize
double word concatenation, so they handle sign_extend the same way that
they handle zero_extend in appropriate contexts.

As a motivating example consider the following function:

__int128 foo(long long x, unsigned long long y)
{
  return ((__int128)x<<64) | y;
}

when compiled with -O2, x86_64 currently generates:

foo:	movq    %rdi, %rdx
        xorl    %eax, %eax
        xorl    %edi, %edi
        orq     %rsi, %rax
        orq     %rdi, %rdx
        ret

with this patch we now generate (the same as if x is unsigned):

foo:	movq    %rsi, %rax
        movq    %rdi, %rdx
        ret

Treating both extensions the same way using any_extend is valid as
the top (extended) bits are "unused" after the shift by 64 (or more).
In theory, the RTL optimizers might consider canonicalizing the form
of extension used in these cases, but zero_extend is faster on some
machine, whereas sign extension is supported via addressing modes on
others, so handling both in the machine description is probably best.

2024-06-28  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386.md (*concat<mode><dwi>3_3): Change zero_extend
	to any_extend in first operand to left shift by mode precision.
	(*concat<mode><dwi>3_4): Likewise.
	(*concat<mode><dwi>3_6): Likewise.

gcc/testsuite/ChangeLog
	* gcc.target/i386/concatditi-1.c: New test case.