load_unit: Fix #356 uncached loads served from WB

Created by: zarubaf

This is a tentative fix for #356 (closed). I would very much appreciate your feedback @jrrk as I don't have an isolated test case for it.

What the fix does is basically checking whether the write buffer is empty in case we are performing an uncached load. If the WB should not be empty the load is killed and retried once the WB gets empty.