[llvm][SveEmitter] Emit the bfloat version of `svld1ro`.

Summary:
The new SVE builtin type __SVBFloat16_t` is used to represent scalable
vectors of bfloat elements.

Reviewers: sdesmalen, efriedma, stuij, ctetreau, shafik, rengolin

Subscribers: tschuett, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81304