[SVE][CodeGen] Lower scalable floating-point vector reductions

Changes in this patch:
-  Minor changes to the LowerVECREDUCE_SEQ_FADD function added by @cameron.mcinally
   to also work for scalable types
- Added TableGen patterns for FP reductions with unpacked types (nxv2f16, nxv4f16 & nxv2f32)
- Asserts added to expandFMINNUM_FMAXNUM & expandVecReduceSeq for scalable types

Reviewed By: cameron.mcinally

Differential Revision: https://reviews.llvm.org/D93050