glsl: In lower_jumps.cpp, lower both branches of a conditional.

glsl: In lower_jumps.cpp, lower both branches of a conditional.

Previously, lower_jumps.cpp would break out of its loop after lowering
a jump instruction in just the then- or else-branch of a conditional,
and it would fail to lower a jump instruction occurring in the other
branch.

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_returns_4.