Add static asserts for MAX_BACKENDS limiting factors

So far the various dependencies were documented in the comment above
MAX_BACKENDS, but not checked.

Discussion: https://postgr.es/m/CA+COZaBO_s3LfALq=b+HcBHFSOEGiApVjrRacCe4VP9m7CJsNQ@mail.gmail.com

src/backend/storage/lmgr/deadlock.c

@@ -30,6 +30,7 @@
 #include "pgstat.h"
 #include "storage/lmgr.h"
 #include "storage/proc.h"
+#include "storage/procnumber.h"
 #include "utils/memutils.h"
 
 
@@ -191,6 +192,8 @@ InitDeadLockChecking(void)
 	 * last MaxBackends entries in possibleConstraints[] are reserved as
 	 * output workspace for FindLockCycle.
 	 */
+	StaticAssertStmt(MAX_BACKENDS_BITS <= (32 - 3),
+					 "MAX_BACKENDS_BITS too big for * 4");
 	maxPossibleConstraints = MaxBackends * 4;
 	possibleConstraints =
 		(EDGE *) palloc(maxPossibleConstraints * sizeof(EDGE));