Fix matrix dimensioning

Newb GL mistake: matrices in GL are column-major.  This means that
vector_elements is the number of rows.  Making these changes causes
matrix-08.glsl to pass.
diff --git a/ir_function.cpp b/ir_function.cpp
index 1ff5b20..b6139c4 100644
--- a/ir_function.cpp
+++ b/ir_function.cpp
@@ -38,7 +38,7 @@
    case GLSL_TYPE_FLOAT:
    case GLSL_TYPE_BOOL:
       if ((a->vector_elements != b->vector_elements)
-	  || (a->matrix_rows != b->matrix_rows))
+	  || (a->matrix_columns != b->matrix_columns))
 	 return -1;
 
       /* There is no implicit conversion to or from bool.