v3d: Avoid assertion failures when removing end-of-shader instructions.

v3d: Avoid assertion failures when removing end-of-shader instructions.

After generating VIR, we leave c->cursor pointing at the end of the
shader.  If the shader had dead code at the end (for example from preamble
instructions in a shader with no side effects), we would assertion fail
that we were leaving the cursor pointing at freed memory.  Since anything
following DCE should be setting up a new cursor anyway, just clear the
cursor at the start.