[SVE][LoopVectorize] Fix crash in InnerLoopVectorizer::widenPHIInstruction

There were a few places in widenPHIInstruction where calculations of
offsets were failing to take the runtime calculation of VF into
account for scalable vectors. I've fixed those cases in this patch
as well as adding an assert that we should not be scalarising for
scalable vectors.

Tests are added here:

  Transforms/LoopVectorize/AArch64/sve-widen-phi.ll

Differential Revision: https://reviews.llvm.org/D99254