aarch64: Improve popcount for bytes [PR113042]

aarch64: Improve popcount for bytes [PR113042]

For popcount for bytes, we don't need the reduction addition
after the vector cnt instruction as we are only counting one
byte's popcount.
This changes the popcount extend to cover all ALLI rather than GPI.

Changes since v1:
* v2 - Use ALLI iterator and combine all into one pattern.
       Add new testcases popcnt[6-8].c.
* v3 - Simplify TARGET_CSSC path.
       Use convert_to_mode instead of gen_zero_extend* directly.
       Some other small cleanups.

Bootstrapped and tested on aarch64-linux-gnu with no regressions.

	PR target/113042

gcc/ChangeLog:

	* config/aarch64/aarch64.md (popcount<mode>2): Update pattern
	to support ALLI modes.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/popcnt5.c: New test.
	* gcc.target/aarch64/popcnt6.c: New test.
	* gcc.target/aarch64/popcnt7.c: New test.
	* gcc.target/aarch64/popcnt8.c: New test.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>