i386: Implement .SAT_TRUNC for unsigned integers

The following testcase:

unsigned short foo (unsigned int x)
{
  _Bool overflow = x > (unsigned int)(unsigned short)(-1);
  return ((unsigned short)x | (unsigned short)-overflow);
}

currently compiles (-O2) to:

foo:
	xorl	%eax, %eax
	cmpl	$65535, %edi
	seta	%al
	negl	%eax
	orl	%edi, %eax
	ret

We can expand through ustrunc{m}{n}2 optab to use carry flag from the
comparison and generate code using SBB:

foo:
	cmpl	$65535, %edi
	sbbl	%eax, %eax
	orl	%edi, %eax
	ret

or CMOV instruction:

foo:
	movl	$65535, %eax
	cmpl	%eax, %edi
	cmovnc	%edi, %eax
	ret

gcc/ChangeLog:

	* config/i386/i386.md (@cmp<mode>_1): Use SWI mode iterator.
	(ustruncdi<mode>2): New expander.
	(ustruncsi<mode>2): Ditto.
	(ustrunchiqi2): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/sattrunc-1.c: New test.

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