i386: Improve reg pressure of double word right shift then truncate.

i386: Improve reg pressure of double word right shift then truncate.

This patch improves register pressure during reload, inspired by PR 97756.
Normally, a double-word right-shift by a constant produces a double-word
result, the highpart of which is dead when followed by a truncation.
The dead code calculating the high part gets cleaned up post-reload, so
the issue isn't normally visible, except for the increased register
pressure during reload, sometimes leading to odd register assignments.
Providing a post-reload splitter, which clobbers a single wordmode
result register instead of a doubleword result register, helps (a bit).

An example demonstrating this effect is:

unsigned long foo (__uint128_t n)
{
  unsigned long a = n & MASK60;
  unsigned long b = (n >> 60);
  b = b & MASK60;
  unsigned long c = (n >> 120);
  return a+b+c;
}

which currently with -O2 generates (13 instructions):
foo:    movabsq $1152921504606846975, %rcx
        xchgq   %rdi, %rsi
        movq    %rsi, %rax
        shrdq   $60, %rdi, %rax
        movq    %rax, %rdx
        movq    %rsi, %rax
        movq    %rdi, %rsi
        andq    %rcx, %rax
        shrq    $56, %rsi
        andq    %rcx, %rdx
        addq    %rsi, %rax
        addq    %rdx, %rax
        ret

with this patch, we generate one less mov (12 instructions):
foo:    movabsq $1152921504606846975, %rcx
        xchgq   %rdi, %rsi
        movq    %rdi, %rdx
        movq    %rsi, %rax
        movq    %rdi, %rsi
        shrdq   $60, %rdi, %rdx
        andq    %rcx, %rax
        shrq    $56, %rsi
        addq    %rsi, %rax
        andq    %rcx, %rdx
        addq    %rdx, %rax
        ret

The significant difference is easier to see via diff:
<       shrdq   $60, %rdi, %rax
<       movq    %rax, %rdx
---
>       shrdq   $60, %rdi, %rdx

Admittedly a single "mov" isn't much of a saving on modern architectures,
but as demonstrated by the PR, people still track the number of them.

2023-11-13  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386.md (<insn><dwi>3_doubleword_lowpart): New
	define_insn_and_split to optimize register usage of doubleword
	right shifts followed by truncation.