[AArch64] Unify lowering logic for fixed-length vectors. (#89393)

In preparation of decoupling codegen for SME from SVE, this patch does a
bit of cleanup to unify the logic around calling
'addTypeForFixedLengthSVE'.

We only want to call this function when:
* We have access to both SVE and NEON, but we prefer to use SVE.
* We have access to SVE, but there is no access to NEON.

Inside 'addTypeForFixedLengthSVE', we normally use Custom lowering for
all operations so they can be converted to/from scalable vector
operations.

However, there are some exceptions:

* For 64/128bit vector loads/stores we prefer the AdvSIMD LDR/STR
D/Q-reg instructions, since these are available in Streaming-SVE mode.

* For some operations like gather/scatter, we can only use SVE if the
full set of SVE instructions is available (as opposed to the
streaming[-compatible] subset). Otherwise, these operations need to
expand (scalarise)