[AArch64][SME] Create new interface for isSVEAvailable.

When a function is compiled to be in Streaming(-compatible) mode, the full
set of SVE instructions may not be available. This patch adds an interface
to query that and changes the codegen for FADDA (not legal in Streaming-SVE
mode) to instead be expanded for fixed-length vectors, or otherwise not to
code-generate for scalable vectors.

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D156109