[SveEmitter] Add support for _n form builtins

The ACLE has builtins that take a scalar value that is to be expanded
into a vector by the operation. While the ISA may have an instruction
that takes an immediate or a scalar to represent this, the LLVM IR
intrinsic may not, so Clang will have to splat the scalar value.

This patch also adds the _n forms for svabd, svadd, svdiv, svdivr,
svmax, svmin, svmul, svmulh, svub and svsubr.

Reviewers: SjoerdMeijer, efriedma, rovka

Reviewed By: SjoerdMeijer

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77594