LSH : Load Sign-extended Half-word (and update the pointer)

This instruction extracts one half-word (16 bits) from a given register, emulating the "Load Halfword" instruction of Load-Store architectures. The data is shifted right, according to an offset, and the MSB are filled with the sign bit of the extracted data.

If the target register is a static register, then the offset is zero. If this is the Data register of a PFQ, the 2 LSBs of the associated pointer are used as offset, and the pointer is updated.

Because this instruction can't go beyond a word boundary, only offsets 0, 1 and 2 are valid. Attempts to use an offset of 3 triggers an alignment trap.