glsl: Handle empty if statement encountered during loop analysis.

glsl: Handle empty if statement encountered during loop analysis.

The is_loop_terminator() function was asserting that the following
kind of if statement could never occur:

    if (...) { } else { }

(presumably based on the assumption that such an if statement would be
eliminated by previous optimization stages).  But that isn't the
case--it's possible that previous optimization stages might simplify
more complex code down to this empty if statement, in which case it
won't be eliminated until the next time through the optimization loop.

So is_loop_terminator() needs to handle it.  Fortunately it's easy to
handle--it's not a loop terminator because it does nothing.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=64330
CC: mesa-stable@lists.freedesktop.org
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>