[AArch64] Extending lowering of 'trunc <(8|16) x i64> %x to <(8|16) x i8>' to use tbl instructions

[AArch64] Patch for lowering trunc instructions to 'tbl' for (8|16)xi32 -> (8|16)xi8 conversions in https://reviews.llvm.org/D133495 is extended to support trunc to tbl lowering for (8|16) x i64 to (8|16) x i8.

A microbenchmark for runtime for these transformations is added in https://reviews.llvm.org/D136274

Reviewed by: fhahn, t.p.northover

Differential Revision: https://reviews.llvm.org/D135229