JIT tuple deforming in LLVM JIT provider.

Performing JIT compilation for deforming gains performance benefits
over unJITed deforming from compile-time knowledge of the tuple
descriptor. Fixed column widths, NOT NULLness, etc can be taken
advantage of.

Right now the JITed deforming is only used when deforming tuples as
part of expression evaluation (and obviously only if the descriptor is
known). It's likely to be beneficial in other cases, too.

By default tuple deforming is JITed whenever an expression is JIT
compiled. There's a separate boolean GUC controlling it, but that's
expected to be primarily useful for development and benchmarking.

Docs will follow in a later commit containing docs for the whole JIT
feature.

Author: Andres Freund
Discussion: https://postgr.es/m/20170901064131.tazjxwus3k2w3ybh@alap3.anarazel.de

src/include/nodes/execnodes.h

@@ -920,6 +920,7 @@ typedef struct PlanState
 	ExprState  *qual;			/* boolean qual condition */
 	struct PlanState *lefttree; /* input plan tree(s) */
 	struct PlanState *righttree;
+
 	List	   *initPlan;		/* Init SubPlanState nodes (un-correlated expr
 								 * subselects) */
 	List	   *subPlan;		/* SubPlanState nodes in my expressions */
@@ -935,6 +936,13 @@ typedef struct PlanState
 	TupleTableSlot *ps_ResultTupleSlot; /* slot for my result tuples */
 	ExprContext *ps_ExprContext;	/* node's expression-evaluation context */
 	ProjectionInfo *ps_ProjInfo;	/* info for doing tuple projection */
+
+	/*
+	 * Scanslot's descriptor if known. This is a bit of a hack, but otherwise
+	 * it's hard for expression compilation to optimize based on the
+	 * descriptor, without encoding knowledge about all executor nodes.
+	 */
+	TupleDesc	scandesc;
 } PlanState;
 
 /* ----------------