fix the wrong zero extend instruction handling

fix the wrong zero extend instruction handling

This helps to fix the problem facing Xing Homer. When we encounter 'zext'
instruction, we should take the soruce type as an unsigned type, otherwise
we're just sign extend the number, which is not what we want.

For the implementation, I'm reusing getUnsignedType() and this expects an
integer to be accepted, and this conforms to the spec of LLVM.

This patch doesn't trigger regression. I'll provide the corresponding test case
later.

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