Fix wrong code due to incorrect define_split

Fix wrong code due to incorrect define_split

-(define_split
-  [(set (match_operand:V2HI 0 "register_operand")
-        (eq:V2HI
-          (eq:V2HI
-            (us_minus:V2HI
-              (match_operand:V2HI 1 "register_operand")
-              (match_operand:V2HI 2 "register_operand"))
-            (match_operand:V2HI 3 "const0_operand"))
-          (match_operand:V2HI 4 "const0_operand")))]
-  "TARGET_SSE4_1"
-  [(set (match_dup 0)
-        (umin:V2HI (match_dup 1) (match_dup 2)))
-   (set (match_dup 0)
-        (eq:V2HI (match_dup 0) (match_dup 2)))])

the splitter is wrong when op1 == op2.(the original pattern returns 0, after split, it returns 1)
So remove the splitter.

Also extend another define_split to define_insn_and_split to handle
below pattern

494(set (reg:V4QI 112)
495    (unspec:V4QI [
496            (subreg:V4QI (reg:V2HF 111 [ bf ]) 0)
497            (subreg:V4QI (reg:V2HF 110 [ af ]) 0)
498            (subreg:V4QI (eq:V2HI (eq:V2HI (reg:V2HI 105)
499                        (const_vector:V2HI [
500                                (const_int 0 [0]) repeated x2
501                            ]))
502                    (const_vector:V2HI [
503                            (const_int 0 [0]) repeated x2
504                        ])) 0)
505        ] UNSPEC_BLENDV))

define_split doesn't work since pass_combine assume it produces at
most 2 insns after split, but here it produces 3 since we need to move
const0_rtx (V2HImode) to reg. The move insn can be eliminated later.

gcc/ChangeLog:

	PR target/112276
	* config/i386/mmx.md (*mmx_pblendvb_v8qi_1): Change
	define_split to define_insn_and_split to handle
	immediate_operand for comparison.
	(*mmx_pblendvb_v8qi_2): Ditto.
	(*mmx_pblendvb_<mode>_1): Ditto.
	(*mmx_pblendvb_v4qi_2): Ditto.
	(<code><mode>3): Remove define_split after it.
	(<code>v8qi3): Ditto.
	(<code><mode>3): Ditto.
	(<ode>v2hi3): Ditto.

gcc/testsuite/ChangeLog:

	* g++.target/i386/part-vect-vcondhf.C: Adjust testcase.
	* gcc.target/i386/pr112276.c: New test.