glsl: lower unconditional returns and continues in loops.

glsl: lower unconditional returns and continues in loops.

Previously, lower_jumps.cpp would only lower return and continue
statements that appeared inside conditionals.  This patch makes it
lower unconditional returns and continue statements that occur inside
a loop.

Such unconditional flow control statements would be unlikely to be
explicitly coded by a reasonable user, however they might arise as a
result of other optimizations.

Without this patch, lower_jumps.cpp might not lower certain return and
continue statements, causing some backends to fail.

Fixes unit tests test_lower_return_void_at_end_of_loop and
test_remove_continue_at_end_of_loop.