[SVE] Code generation for fixed length vector loads & stores.

Summary:
This patch adds base support for code generating fixed length
vector operations targeting a known SVE vector length. To achieve
this we lower fixed length vector operations to equivalent scalable
vector operations, whereby SVE predication is used to limit the
elements processed to those present within the fixed length vector.

Specifically this patch implements load and store operations, which
get lowered to their masked counterparts thusly:

  V = load(Addr) =>
    V = extract_fixed_vector(masked_load(make_pred(V.NumElts), Addr))

  store(V, (Addr)) =>
    masked_store(insert_fixed_vector(V), make_pred(V.NumElts), Addr))

Reviewers: rengolin, efriedma

Subscribers: tschuett, hiraditya, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80385