glsl2: Don't dead-code eliminate a call where the return value is unused.

This showed up since the disabling of inlining at compile time, which
I apparently didn't regenerate piglit summary for.

Fixes:
glsl-deadcode-call.
diff --git a/src/glsl/ir_dead_code.cpp b/src/glsl/ir_dead_code.cpp
index a8d264f..8798887 100644
--- a/src/glsl/ir_dead_code.cpp
+++ b/src/glsl/ir_dead_code.cpp
@@ -78,7 +78,8 @@
 	  * Don't do so if it's a shader output, though.
 	  */
 	 if (entry->var->mode != ir_var_out &&
-	     entry->var->mode != ir_var_inout) {
+	     entry->var->mode != ir_var_inout &&
+	     !ir_has_call(entry->assign)) {
 	    entry->assign->remove();
 	    progress = true;