Inline hot path of slot_getsomeattrs().

This yields a minor speedup, which roughly balances the loss from the
upcoming introduction of callbacks to do some operations on slots.

Author: Andres Freund
Discussion: https://postgr.es/m/20181105210039.hh4vvi4vwoq5ba2q@alap3.anarazel.de

src/backend/executor/execExprInterp.c

@@ -428,7 +428,6 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
 		{
 			CheckOpSlotCompatibility(op, innerslot);
 
-			/* XXX: worthwhile to check tts_nvalid inline first? */
 			slot_getsomeattrs(innerslot, op->d.fetch.last_var);
 
 			EEO_NEXT();