This attribute represents the minimum and maximum values vscale can
take. For now this attribute is not hooked up to anything during
codegen, this will be added in the future when such codegen is
considered stable.
Additionally hook up the -msve-vector-bits=<x> clang option to emit this
attribute.
I'm happy with this but for information you could use the same trick as for the llvm/test/Analysis/CostModel/AArch64/sve-fixed-length.ll so that only a single CHECK, and CHECK-NOT line is required.