x86: Extend AVX512 Vectorization for Popcount in Various Modes

This patch enables vectorization of the popcount operation for V2QI, V4QI,
V8QI, V2HI, V4HI, and V2SI modes.

gcc/ChangeLog:

	* config/i386/mmx.md:
	(VQI_16_32_64): New mode iterator for 8-byte, 4-byte, and 2-byte QImode.
	(popcount<mode>2): New pattern for popcount of V2QI/V4QI/V8QI mode.
	(popcount<mode>2): New pattern for popcount of V2HI/V4HI mode.
	(popcountv2si2): New pattern for popcount of V2SI mode.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/part-vect-popcount-1.c: New test.