glsl: Teach basic block analysis about break/continue/discard.

Previously, the only kind of ir_jump that would terminate a basic
block was "return".  However, the other possible types of ir_jump
("break", "continue", and "discard") should terminate a basic block
too.  This patch modifies basic block analysis so that it terminates a
basic block on any type of ir_jump, not just ir_return.

Fixes piglit test dead-code-break-interaction.shader_test.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/glsl/ir_basic_block.cpp

@@ -77,7 +77,7 @@ void call_for_basic_blocks(exec_list *instructions,
 	 callback(leader, ir, data);
 	 leader = NULL;
 	 call_for_basic_blocks(&ir_loop->body_instructions, callback, data);
-      } else if (ir->as_return() || ir->as_call()) {
+      } else if (ir->as_jump() || ir->as_call()) {
 	 callback(leader, ir, data);
 	 leader = NULL;
       } else if ((ir_function = ir->as_function())) {