add helper functions in ir::Constant and ir::ConstantSet

add helper functions in ir::Constant and ir::ConstantSet

Version 3.
Deleted empty destructor of ConstantSet.
Updated ConstantSet::getConstant()

And I am afraid I cannot delete the empty constructor of ConstantSet.
That will break the constructor of ir::Unit. See unit.cpp, line 30.

add ir::Constant.reg field
add helper functions in ir::ConstantSet to get specified ir::Constant
and binary packed constant array

Signed-off-by: Homer Hsing <homer.xing@intel.com>
Reviewed-by: Zhigang Gong <zhigang.gong@linux.intel.com>