Some tweaks to the SVE2p1 load and store intrinsics

[rsandifo-arm]

The pre-SVE2p1 gather and scatter intrinsics allow vector displacements (offsets or indices) to be either signed or unsigned. svld1q and svst1q instead required them to be unsigned. This patch adds signed versions too, for consistency.

Also, the SVE2p1 stores were specified to take pointers to const, but they ought to be pointers to non-const instead.

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