gc.c: reduce size slightly by making MEMOP_TYPE_MALLOC zero

Most (if not all) architectures have instructions for comparing
against zero, allowing compilers to generate more compact code.

Other MEMOP_TYPE_* enum values are not compared in hot paths,
but MEMOP_TYPE_MALLOC is checked in objspace_malloc_increase

   text	   data	    bss	    dec	    hex	filename
  84088	    264	   3664	  88016	  157d0	gc-before.o
  83784	    264	   3664	  87712	  156a0	gc.o

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63546 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

gc.c

@@ -7785,9 +7785,9 @@ objspace_malloc_size(rb_objspace_t *objspace, void *ptr, size_t hint)
 }
 
 enum memop_type {
-    MEMOP_TYPE_MALLOC  = 1,
-    MEMOP_TYPE_FREE    = 2,
-    MEMOP_TYPE_REALLOC = 3
+    MEMOP_TYPE_MALLOC  = 0,
+    MEMOP_TYPE_FREE,
+    MEMOP_TYPE_REALLOC
 };
 
 static inline void