[SVE][LoopVectorize] Optimise code generated by widenPHIInstruction

For SVE, when scalarising the PHI instruction the whole vector part is
generated as opposed to creating instructions for each lane for fixed-
width vectors. However, in some cases the lane values may be needed
later (e.g for a load instruction) so we still need to calculate
these values to avoid extractelement being called on the vector part.

Differential Revision: https://reviews.llvm.org/D109445