x86_64: Improve code expanded for highpart multiplications.

While working on a middle-end patch to more aggressively use highpart
multiplications on targets that support them, I noticed that the RTL
expanded by the x86 backend interacts poorly with register allocation
leading to suboptimal code.

For the testcase,
typedef int __attribute ((mode(TI))) ti_t;
long foo(long x)
{
  return ((ti_t)x * 19065) >> 64;
}

we'd like to avoid:
foo:	movq    %rdi, %rax
        movl    $19065, %edx
        imulq   %rdx
        movq    %rdx, %rax
        ret

and would prefer:
foo:	movl    $19065, %eax
        imulq   %rdi
        movq    %rdx, %rax
        ret

This patch provides a pair of peephole2 transformations to tweak the
spills generated by reload, and at the same time replaces the current
define_expand with a define_insn pattern using the new [su]mul_highpart
RTX codes.

2021-12-20  Roger Sayle  <roger@nextmovesoftware.com>
	    Uroš Bizjak  <ubizjak@gmail.com>

gcc/ChangeLog
	* config/i386/i386.md (any_mul_highpart): New code iterator.
	(sgnprefix, s): Add attribute support for [su]mul_highpart.
	(<s>mul<mode>3_highpart): Delete expander.
	(<s>mul<mode>3_highpart, <s>mulsi32_highpart_zext):
	New define_insn patterns.
	(define_peephole2): Tweak the register allocation for the above
	instructions after reload.

gcc/testsuite/ChangeLog
	* gcc.target/i386/smuldi3_highpart.c: New test case.