gallivm: fix return opcode handling in main function of a shader

gallivm: fix return opcode handling in main function of a shader

If we're in some conditional or loop we must not return, or the code
after the condition is never executed.
(v2): And, we also can't just continue as nothing happened, since the
mask update code would later check if we actually have a mask, so we
need to remember that there was a return in main where we didn't exit
(to illustrate this, a ret in a if clause would cause a mask update
which is still ok as we're in a conditional, but after the endif the
mask update code would drop the mask hence bringing execution back to
pixels which should have their execution mask set to zero by the ret).
Thanks to Christoph Bumiller for figuring this out.

This fixes https://bugs.freedesktop.org/show_bug.cgi?id=62357.

Note: This is a candidate for the stable branches.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>