Combine two flag tests in GetSnapshotData().

Previously the code checked PROC_IN_LOGICAL_DECODING and
PROC_IN_VACUUM separately. As the relevant variable is marked as
volatile, the compiler cannot combine the two tests.  As
GetSnapshotData() is pretty hot in a number of workloads, it's
worthwhile to fix that.

It'd also be a good idea to get rid of the volatiles altogether. But
for one that's a larger patch, and for another, the code after this
change still seems at least as easy to read as before.

Author: Andres Freund
Discussion: https://postgr.es/m/20181005172955.wyjb4fzcdzqtaxjq@alap3.anarazel.de

src/backend/storage/ipc/procarray.c

@@ -1589,14 +1589,11 @@ GetSnapshotData(Snapshot snapshot)
 			TransactionId xid;
 
 			/*
-			 * Backend is doing logical decoding which manages xmin
-			 * separately, check below.
+			 * Skip over backends doing logical decoding which manages xmin
+			 * separately (check below) and ones running LAZY VACUUM.
 			 */
-			if (pgxact->vacuumFlags & PROC_IN_LOGICAL_DECODING)
-				continue;
-
-			/* Ignore procs running LAZY VACUUM */
-			if (pgxact->vacuumFlags & PROC_IN_VACUUM)
+			if (pgxact->vacuumFlags &
+				(PROC_IN_LOGICAL_DECODING | PROC_IN_VACUUM))
 				continue;
 
 			/* Update globalxmin to be the smallest valid xmin */