glsl: Lower break instructions when necessary at the end of a loop.

glsl: Lower break instructions when necessary at the end of a loop.

Normally lower_jumps.cpp doesn't need to lower a break instruction
that occurs at the end of a loop, because all back-ends can produce
proper GPU instructions for a break instruction in this "canonical"
location.  However, if other break instructions within the loop are
already being lowered, then a break instruction at the end of the loop
needs to be lowered too, since after the optimization is complete a
new conditional break will be inserted at the end of the loop.

Without this patch, lower_jumps.cpp may require multiple passes in
order to lower all jumps.  This results in sub-optimal output because
lower_jumps.cpp produces a brand new set of temporary variables each
time it is run, and the redundant temporary variables are not
guaranteed to be eliminated by later optimization passes.

Fixes unit test test_lower_breaks_6.