glsl: Refactor logic for determining whether to lower return statements.

glsl: Refactor logic for determining whether to lower return statements.

Previously, do_lower_jumps.cpp determined whether to lower return
statements in ir_lower_jumps_visitor::should_lower_jumps().  Moved
this logic to ir_lower_jumps_visitor::visit(ir_function_signature *),
so that it can be used in determining whether to lower a return
statement at the end of a function.