[SVE] Add lowering for scalable vector fadd, fdiv, fmul and fsub operations.

Lower the operations to predicated variants.  This is prep work
required for fixed length code generation but also fixes a bug
whereby these operations fail selection when "unpacked" vector
types (e.g. MVT::nxv2f32) are used.

This patch also adds the missing "unpacked" patterns for FMA.

Differential Revision: https://reviews.llvm.org/D83765