i386: Improve code generation of smin(x,0) with -m32.

i386: Improve code generation of smin(x,0) with -m32.

To make amends for the recent (temporary) testsuite failure
of my new gcc.target/i386/minmax-9.c when compiled with -m32,
this patch improves the -m32 code we generate for the examples
in that test case.

The trick is to expand smin(x,0) as "x < 0 ? x : 0" instead
of the current "x <= 0 ? x : 0", as the former can take
advantage of sign_bit_mask operations.

2020-08-10  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386-expand.c (ix86_expand_int_movcc): Expand
	signed MIN_EXPR against zero as "x < 0 ? x : 0" instead of
	"x <= 0 ? x : 0" to enable sign_bit_compare_p optimizations.

gcc/testsuite/ChangeLog
	* gcc.target/i386/minmax-12.c: New test.