[AArch64][ISel] Extend vector_splice tests (NFC)

They use extract shuffles for fixed vectors, and
llvm.vector.splice intrinsics for scalable vectors.

In the previous tests using ld+extract+st, the extract was optimized
away and replaced by a smaller load at the right offset. This meant
we didin't really test the vector_splice ISD node.

Author: gbossu