[AArch64][sve] Prevent incorrect function call on fixed width vector

The isEssentiallyExtractHighSubvector function currently calls
getVectorNumElements on a type that in specific cases might be scalable.
Since this function only has correct behaviour at the moment on scalable
types anyway, the function can just return false when given a fixed type.

Differential Revision: https://reviews.llvm.org/D109163