[AArch64][SVE] Ensure LowerEXTRACT_SUBVECTOR is not called for illegal types

The lowering for EXTRACT_SUBVECTOR should not be called during type
legalization, only as part of lowering, hence return SDValue() when
called on illegal types.

This also adds missing tests for extracting fixed types from illegal
scalable types.

Differential Revision: https://reviews.llvm.org/D111412