[clang][AArch64][SVE] Avoid going through memory for coerced VLST return values

VLST return values are coerced to VLATs in the function epilog for
consistency with the VLAT ABI. Previously, this coercion was done
through memory. It is preferable to use the
llvm.experimental.vector.insert intrinsic to avoid going through memory
here.

Reviewed By: c-rhodes

Differential Revision: https://reviews.llvm.org/D94290