Move interrupt checking from ExecProcNode() to executor nodes.

In a followup commit ExecProcNode(), and especially the large switch
it contains, will largely be replaced by a function pointer directly
to the correct node. The node functions will then get invoked by a
thin inline function wrapper. To avoid having to include miscadmin.h
in headers - CHECK_FOR_INTERRUPTS() - move the interrupt checks into
the individual executor routines.

While looking through all executor nodes, I noticed a number of
arguably missing interrupt checks, add these too.

Author: Andres Freund, Tom Lane
Reviewed-By: Tom Lane
Discussion:
    https://postgr.es/m/22833.1490390175@sss.pgh.pa.us

src/backend/executor/nodeIndexscan.c

@@ -34,6 +34,7 @@
 #include "executor/execdebug.h"
 #include "executor/nodeIndexscan.h"
 #include "lib/pairingheap.h"
+#include "miscadmin.h"
 #include "nodes/nodeFuncs.h"
 #include "optimizer/clauses.h"
 #include "utils/array.h"
@@ -131,6 +132,8 @@ IndexNext(IndexScanState *node)
 	 */
 	while ((tuple = index_getnext(scandesc, direction)) != NULL)
 	{
+		CHECK_FOR_INTERRUPTS();
+
 		/*
 		 * Store the scanned tuple in the scan tuple slot of the scan state.
 		 * Note: we pass 'false' because tuples returned by amgetnext are
@@ -233,6 +236,8 @@ IndexNextWithReorder(IndexScanState *node)
 
 	for (;;)
 	{
+		CHECK_FOR_INTERRUPTS();
+
 		/*
 		 * Check the reorder queue first.  If the topmost tuple in the queue
 		 * has an ORDER BY value smaller than (or equal to) the value last
@@ -299,6 +304,8 @@ next_indextuple:
 			{
 				/* Fails recheck, so drop it and loop back for another */
 				InstrCountFiltered2(node, 1);
+				/* allow this loop to be cancellable */
+				CHECK_FOR_INTERRUPTS();
 				goto next_indextuple;
 			}
 		}