Move pattern selectivity code from selfuncs.c to like_support.c.

While at it, refactor patternsel() a bit so that it can be used from
the LIKE/regex planner support functions as well.  This makes the
planner able to deal equally well with either operator or function
syntax for these operations.  I'm not excited about that as a feature
in itself, but it provides a nice model for extensions to follow if
they want such behavior for their operations.

This change localizes the use of pattern_fixed_prefix() and
make_greater_string() so that they no longer need be exported.
(We might get pushback from extensions about that, perhaps,
in which case I'd be inclined to re-export them in a new header
file like_support.h.)

This reduces the bulk of selfuncs.c a fair amount, removing ~1370
lines or about one-sixth of that file; it's still too big, but this
is progress.

Discussion: https://postgr.es/m/24537.1550093915@sss.pgh.pa.us

src/include/utils/selfuncs.h

@@ -15,7 +15,6 @@
 #ifndef SELFUNCS_H
 #define SELFUNCS_H
 
-#include "fmgr.h"
 #include "access/htup.h"
 #include "nodes/pathnodes.h"
 
@@ -85,20 +84,6 @@ typedef struct VariableStatData
 	} while(0)
 
 
-typedef enum
-{
-	Pattern_Type_Like,
-	Pattern_Type_Like_IC,
-	Pattern_Type_Regex,
-	Pattern_Type_Regex_IC,
-	Pattern_Type_Prefix
-} Pattern_Type;
-
-typedef enum
-{
-	Pattern_Prefix_None, Pattern_Prefix_Partial, Pattern_Prefix_Exact
-} Pattern_Prefix_Status;
-
 /*
  * deconstruct_indexquals is a simple function to examine the indexquals
  * attached to a proposed IndexPath.  It returns a list of IndexQualInfo
@@ -175,14 +160,16 @@ extern double histogram_selectivity(VariableStatData *vardata, FmgrInfo *opproc,
 					  Datum constval, bool varonleft,
 					  int min_hist_size, int n_skip,
 					  int *hist_size);
-
-extern Pattern_Prefix_Status pattern_fixed_prefix(Const *patt,
-					 Pattern_Type ptype,
-					 Oid collation,
-					 Const **prefix,
-					 Selectivity *rest_selec);
-extern Const *make_greater_string(const Const *str_const, FmgrInfo *ltproc,
-					Oid collation);
+extern double ineq_histogram_selectivity(PlannerInfo *root,
+						   VariableStatData *vardata,
+						   FmgrInfo *opproc, bool isgt, bool iseq,
+						   Datum constval, Oid consttype);
+extern double var_eq_const(VariableStatData *vardata, Oid oproid,
+			 Datum constval, bool constisnull,
+			 bool varonleft, bool negate);
+extern double var_eq_non_const(VariableStatData *vardata, Oid oproid,
+				 Node *other,
+				 bool varonleft, bool negate);
 
 extern Selectivity boolvarsel(PlannerInfo *root, Node *arg, int varRelid);
 extern Selectivity booltestsel(PlannerInfo *root, BoolTestType booltesttype,