[SVE][LoopVectorize] Add support for scalable vectorization of first-order recurrences

Adds support for scalable vectorization of loops containing first-order recurrences, e.g:
```
for(int i = 0; i < n; i++)
  b[i] =  a[i] + a[i - 1]
```
This patch changes fixFirstOrderRecurrence for scalable vectors to take vscale into
account when inserting into and extracting from the last lane of a vector.
CreateVectorSplice has been added to construct a vector for the recurrence, which
returns a splice intrinsic for scalable types. For fixed-width the behaviour
remains unchanged as CreateVectorSplice will return a shufflevector instead.

The tests included here are the same as test/Transform/LoopVectorize/first-order-recurrence.ll

Reviewed By: david-arm, fhahn

Differential Revision: https://reviews.llvm.org/D101076