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));

src/backend/utils/cache/inval.c

@@ -120,6 +120,7 @@
 #include "catalog/catalog.h"
 #include "catalog/pg_constraint.h"
 #include "miscadmin.h"
+#include "storage/procnumber.h"
 #include "storage/sinval.h"
 #include "storage/smgr.h"
 #include "utils/catcache.h"
@@ -1651,6 +1652,10 @@ CacheInvalidateSmgr(RelFileLocatorBackend rlocator)
 {
 	SharedInvalidationMessage msg;
 
+	/* verify optimization stated above stays valid */
+	StaticAssertStmt(MAX_BACKENDS_BITS <= 23,
+					 "MAX_BACKEND_BITS is too big for inval.c");
+
 	msg.sm.id = SHAREDINVALSMGR_ID;
 	msg.sm.backend_hi = rlocator.backend >> 16;
 	msg.sm.backend_lo = rlocator.backend & 0xffff;

src/include/storage/buf_internals.h

@@ -21,6 +21,7 @@
 #include "storage/bufmgr.h"
 #include "storage/condition_variable.h"
 #include "storage/lwlock.h"
+#include "storage/procnumber.h"
 #include "storage/shmem.h"
 #include "storage/smgr.h"
 #include "storage/spin.h"
@@ -86,6 +87,8 @@ StaticAssertDecl(BUF_REFCOUNT_BITS + BUF_USAGECOUNT_BITS + BUF_FLAG_BITS == 32,
 
 StaticAssertDecl(BM_MAX_USAGE_COUNT < (1 << BUF_USAGECOUNT_BITS),
 				 "BM_MAX_USAGE_COUNT doesn't fit in BUF_USAGECOUNT_BITS bits");
+StaticAssertDecl(MAX_BACKENDS_BITS <= BUF_REFCOUNT_BITS,
+				 "MAX_BACKENDS_BITS needs to be <= BUF_REFCOUNT_BITS");
 
 /*
  * Buffer tag identifies which disk block the buffer contains.