[SVE] Add support for scalable vectorization of loops with selects and cmps

I have removed an unnecessary assert in LoopVectorizationCostModel::getInstructionCost
that prevented a cost being calculated for select instructions when using
scalable vectors. In addition, I have changed AArch64TTIImpl::getCmpSelInstrCost
to only do special cost calculations for fixed width vectors and fall
back to the base version for scalable vectors.

I have added a simple cost model test for cmps and selects:

  test/Analysis/CostModel/sve-cmpsel.ll

and some simple tests that show we vectorize loops with cmp and select:

  test/Transforms/LoopVectorize/AArch64/sve-basic-vec.ll

Differential Revision: https://reviews.llvm.org/D95039