[SVE][CodeGen] Legalisation of integer -> floating point conversions

Splitting the operand of a scalable [S|U]INT_TO_FP results in a
concat_vectors operation where the operands are unpacked FP
scalable vectors (e.g. nxv2f32).
This patch adds custom lowering of concat_vectors which
checks that the number of operands is 2, and isel patterns
to match concat_vectors of scalable FP types with uzp1.

Reviewed By: efriedma, paulwalker-arm

Differential Revision: https://reviews.llvm.org/D88033