[AArch64][SVE] Better utilisation of unpredicated forms of arithmetic intrinsics

When using predicated arithmetic intrinsics, if the predicate used is all
lanes active, use an unpredicated form of the instruction, additionally
this allows for better use of immediate forms.

This also includes a new complex isel pattern which allows matching an
all active predicate when the types are different but the predicate is a
superset of the type being used. For example, to allow a b8 ptrue for a
b32 predicate operand.

This only includes instructions where the unpredicated/predicated forms
are mismatched between variants, meaning that the removal of the
predicate is done during instruction selection in order to prevent
spurious re-introductions of ptrue instructions.

Co-authored-by: Paul Walker <paul.walker@arm.com>

Differential Revision: https://reviews.llvm.org/D101062