aarch64: Implement popcountti2 pattern [PR113042]

When CSSC is not enabled, 128bit popcount can be implemented
just via the vector (v16qi) cnt instruction followed by a reduction,
like how the 64bit one is currently implemented instead of
splitting into 2 64bit popcount.

Changes since v1:
* v2: Make operand 0 be DImode instead of TImode and simplify.

Build and tested for aarch64-linux-gnu.

	PR target/113042

gcc/ChangeLog:

	* config/aarch64/aarch64.md (popcountti2): New define_expand.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/popcnt10.c: New test.
	* gcc.target/aarch64/popcnt9.c: New test.

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