[AArch64] Lower fixed-length vector_shuffle to SVE splat if possible

This adds an extra case to check if a vector_shuffle for a fixed-length
vector that's being lowered to SVE, is just a splat. Doing this avoids
a round trip to the stack and back for a few cases.

Reviewed By: c-rhodes

Differential Revision: https://reviews.llvm.org/D137966