[AMDGPU] Add VGPR copies post regalloc fix pass

Regalloc creates COPY instructions which do not formally use VALU.
That results in v_mov instructions displaced after exec mask modification.
One pass which do it is SIOptimizeExecMasking, but potentially it can be
done by other passes too.

This patch adds a pass immediately after regalloc to add implicit exec
use operand to all VGPR copy instructions.

Differential Revision: https://reviews.llvm.org/D28874

llvm-svn: 292956