The macro is emitted when wargeting SVE code generation with the additional command line option `-msve-vector-bits=<N>`.
The behavior implied by the macro is described in sections "3.7.3.3. Behavior specific to SVE vectors" of the SVE ACLE (Version 00bet6) that can be found at https://developer.arm.com/documentation/100987/latest
Reviewed By: rengolin, rsandifo-arm
Differential Revision: https://reviews.llvm.org/D90956
Francesco Petrogalli