[AArch64][SME] Remove immediate argument restriction for svldr and svstr (#68565)

The svldr_vnum and svstr_vnum builtins always modify the base register
and tile slice and provide immediate offsets of zero, even when the
offset provided to the builtin is an immediate. This patch optimises the
output of the builtins when the offset is an immediate, to pass it
directly to the instruction and to not need the base register and tile
slice updates.