[AArch64][SVE2] Shift-Right Accumulate combine patterns

This patch adds combining support for SVE2's shift-right accumulate
instructions.

2019-09-27  Yuliang Wang  <yuliang.wang@arm.com>

gcc/
	* config/aarch64/aarch64-sve2.md (aarch64_sve2_sra<mode>):
	New combine pattern.

gcc/testsuite/
	* gcc.target/aarch64/sve2/shracc_1.c: New test.

From-SVN: r276174