Tweaks to the SME2 intrinsics

[rsandifo-arm]

This patch makes a few tweaks to the SME2 intrinsics:

• The type suffix of the SME2 svread* intrinsics needs to be
  explicit (non-optional), since there is no merge input that
  can be used to infer it.

• The zn argument to the svluti* intrinsics is a collection of
  2-bit and 4-bit quantities, so it doesn't have a natural element
  size or signedness. It seems better to keep it as svuint8_t for
  all variants.

• Because of that, there is no argument that implies the return
  type of the svluti* intrinsics, so the type suffix needs to be
  explicit. Also, since the instruction performs a bag-of-bits
  lookup, it makes sense to have floating-point variants too.

• The ZA slice forms of svread* and svwrite* are likewise
  bag-of-bits moves, so we can provide alternatives for all
  element types.

• arm_neon.h shift-by-immediate instructions use an _n suffix
  to indicate that the shift amount is scalar. arm_sve.h
  carried this across to the full/non-overloaded forms of SVE
  immediate shifts. It seems worth doing the same here for
  consistency, and to protect against vector-vector forms
  being added in future.

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[rsandifo-arm]

cc: @sdesmalen-arm

[sdesmalen-arm]

LGTM

[kmclaughlin-arm]

Hi @rsandifo-arm,
Should the zn argument for the svluti4 x4 also be a svuint8_t?

[sallyarmneale]

No changes