[AMDGPU] Precommit test for i8 vector CopyToReg handling patch

Adds test to show impact on cross block CopyToReg & CopyFromReg handling for n x i8, and shows NFC on CC

Differential Revision: https://reviews.llvm.org/D159303

Change-Id: Ib6d9802dbebe8e3245e4ccfd4a6f23357de8c480

Author: jrbyrnes