AddLower, PairwiseAdd/Sub and MaskedAbsOr operations

[mazimkhan]

Adding special arithmetic operations for arm_sve-inl.h and generic_ops-inl.h:

• AddLower adds the first lane of both input vectors and passes the lanes of vector a for all other lanes.
• PairwiseAdd adds consecutive pairs of elements in each of the vectors and interleaves the resulting lanes.
• PairwiseSub subtracts consecutive pairs of elements in each of the vectors and interleaves the resulting lanes.
• PairwiseAdd128 adds consecutive pairs of elements in each of the vectors and then packs the results in 128 bit blocks, such that the results of vector a are in the lower half of the block and the results of vector b are in the upper half of the block.
• PairwiseSub128 subtracts consecutive pairs of elements in each of the vectors and then packs the results in 128 bit blocks, such that the results of vector a are in the lower half of the block and the results of vector b are in the upper half of the block.

Tests have been added for the operations.

The instruction matrix in g3doc/instruction_matrix.pdf may need to be updated, but it appears to have been generated manually.