Merge two define_insn: <avx512>_blendm<mode>, <avx512>_load<mode>_mask.

Those two define_insns have same pattern, and <avx512>_load<mode>_mask
would always be matched since it show up earlier in the md file, and
it may lose some opportunity in pass_reload since
<avx512>_load<mode>_mask only have constraint "0C" for operand2, and
"v" constraint in <avx512>_vblendm<mode> would never be matched.

2020-07-21  Hongtao Liu  <hongtao.liu@intel.com>

gcc/
	PR target/96246
	* config/i386/sse.md (<avx512>_load<mode>_mask,
	<avx512>_load<mode>_mask): Extend to generate blendm
	instructions.
	(<avx512>_blendm<mode>, <avx512>_blendm<mode>): Change
	define_insn to define_expand.

gcc/testsuite/
	* gcc.target/i386/avx512bw-pr96246-1.c: New test.
	* gcc.target/i386/avx512bw-pr96246-2.c: New test.
	* gcc.target/i386/avx512vl-pr96246-1.c: New test.
	* gcc.target/i386/avx512vl-pr96246-2.c: New test.
	* gcc.target/i386/avx512bw-vmovdqu16-1.c: Adjust test.
	* gcc.target/i386/avx512bw-vmovdqu8-1.c: Ditto.
	* gcc.target/i386/avx512f-vmovapd-1.c: Ditto.
	* gcc.target/i386/avx512f-vmovaps-1.c: Ditto.
	* gcc.target/i386/avx512f-vmovdqa32-1.c: Ditto.
	* gcc.target/i386/avx512f-vmovdqa64-1.c: Ditto.
	* gcc.target/i386/avx512vl-pr92686-movcc-1.c: Ditto.
	* gcc.target/i386/avx512vl-pr96246-1.c: Ditto.
	* gcc.target/i386/avx512vl-pr96246-2.c: Ditto.
	* gcc.target/i386/avx512vl-vmovapd-1.c: Ditto.
	* gcc.target/i386/avx512vl-vmovaps-1.c: Ditto.
	* gcc.target/i386/avx512vl-vmovdqa32-1.c: Ditto.
	* gcc.target/i386/avx512vl-vmovdqa64-1.c: Ditto.