[AArch64][SVE] Fix ICE extracting fixedvec from scalable load

f526c600 had a concern raised because of an invalid typesize request
on a scalable vector, which this patch addresses.

Prevent shouldReduceLoadWidth from attempting to query the bit size, and
add a regression test in sve-extract-fixed-vector.ll.

Differential Revision: https://reviews.llvm.org/D115156