tgsi: Enable returns from within loops

Fixes the following piglit test (for softpipe):
/spec/glsl-1.10/execution/fs-loop-return

Signed-off-by: Lars Hamre <chemecse@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/gallium/auxiliary/tgsi/tgsi_exec.c

@@ -5148,6 +5148,10 @@ exec_instruction(
             /* returning from main() */
             mach->CondStackTop = 0;
             mach->LoopStackTop = 0;
+            mach->ContStackTop = 0;
+            mach->LoopLabelStackTop = 0;
+            mach->SwitchStackTop = 0;
+            mach->BreakStackTop = 0;
             *pc = -1;
             return FALSE;
          }