Fix performance issue in EXPLAIN (ANALYZE, TIMING OFF).

Commit af7914c6, which added the TIMING
option to EXPLAIN, had an oversight: if the TIMING option is disabled
then control in InstrStartNode() goes through an elog(DEBUG2) call, which
typically does nothing but takes a noticeable amount of time to do it.
Tweak the logic to avoid that.

In HEAD, also change the elog(DEBUG2)'s in instrument.c to elog(ERROR).
It's not very clear why they weren't like that to begin with, but this
episode shows that not complaining more vociferously about misuse is
likely to do little except allow bugs to remain hidden.

While at it, adjust some code that was making possibly-dangerous
assumptions about flag bits being in the rightmost byte of the
instrument_options word.

Problem reported by Pavel Stehule (via Tomas Vondra).

src/backend/executor/instrument.c

@@ -33,9 +33,9 @@ InstrAlloc(int n, int instrument_options)
 	instr = palloc0(n * sizeof(Instrumentation));
 	if (instrument_options & (INSTRUMENT_BUFFERS | INSTRUMENT_TIMER))
 	{
+		bool		need_buffers = (instrument_options & INSTRUMENT_BUFFERS) != 0;
+		bool		need_timer = (instrument_options & INSTRUMENT_TIMER) != 0;
 		int			i;
-		bool		need_buffers = instrument_options & INSTRUMENT_BUFFERS;
-		bool		need_timer = instrument_options & INSTRUMENT_TIMER;
 
 		for (i = 0; i < n; i++)
 		{
@@ -51,10 +51,13 @@ InstrAlloc(int n, int instrument_options)
 void
 InstrStartNode(Instrumentation *instr)
 {
-	if (instr->need_timer && INSTR_TIME_IS_ZERO(instr->starttime))
+	if (instr->need_timer)
+	{
+		if (INSTR_TIME_IS_ZERO(instr->starttime))
 			INSTR_TIME_SET_CURRENT(instr->starttime);
 		else
 			elog(DEBUG2, "InstrStartNode called twice in a row");
+	}
 
 	/* save buffer usage totals at node entry, if needed */
 	if (instr->need_bufusage)
@@ -73,7 +76,6 @@ InstrStopNode(Instrumentation *instr, double nTuples)
 	/* let's update the time only if the timer was requested */
 	if (instr->need_timer)
 	{
-
 		if (INSTR_TIME_IS_ZERO(instr->starttime))
 		{
 			elog(DEBUG2, "InstrStopNode called without start");
@@ -84,7 +86,6 @@ InstrStopNode(Instrumentation *instr, double nTuples)
 		INSTR_TIME_ACCUM_DIFF(instr->counter, endtime, instr->starttime);
 
 		INSTR_TIME_SET_ZERO(instr->starttime);
-
 	}
 
 	/* Add delta of buffer usage since entry to node's totals */