x86: Replace ne:CCC/ne:CCO with UNSPEC_CC_NE in neg patterns

In i386.md, neg patterns which set MODE_CC register like

(set (reg:CCC FLAGS_REG)
     (ne:CCC (match_operand:SWI48 1 "general_reg_operand") (const_int 0)))

can lead to errors when operand 1 is a constant value.  If FLAGS_REG in

(set (reg:CCC FLAGS_REG)
     (ne:CCC (const_int 2) (const_int 0)))

is set to 1, RTX simplifiers may simplify

(set (reg:SI 93)
     (neg:SI (ltu:SI (reg:CCC 17 flags) (const_int 0 [0]))))

as

(set (reg:SI 93)
     (neg:SI (ltu:SI (const_int 1) (const_int 0 [0]))))

which leads to incorrect results since LTU on MODE_CC register isn't the
same as "unsigned less than" in x86 backend.  To prevent RTL optimizers
from setting MODE_CC register to a constant, use UNSPEC_CC_NE to replace
ne:CCC/ne:CCO when setting FLAGS_REG in neg patterns.

gcc/

	PR target/107172
	* config/i386/i386.md (UNSPEC_CC_NE): New.
	Replace ne:CCC/ne:CCO with UNSPEC_CC_NE in neg patterns.

gcc/testsuite/

	PR target/107172
	* gcc.target/i386/pr107172.c: New test.