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