Load/Store, masked set and counting operations

[mazimkhan]

Introduces:

• variants of load and store operations including masked variants (MaskedLoadU, LoadHigher, StoreTruncated)
• Counting functions to find information about the data in each lane of a vector (MaskedLeadingZeroCountOrZero, AllOnes, AllZeros)
• Masked vector instantiation operations (SetOr, SetOrZero).

"OrZero" operations will return zero where the mask is false whereas standard masking returns the corresponding lane of a passed vector.

All introduced operations are implemented in generic_ops-inl.h and in arm_sve-inl.h where there is a performance gain to be made. Testing is also performed for all new operations.