Common subexpression elimination in columnar agg (#9534)

1) Use interning to find the identical expression subtrees and replace
   them with a canonical pointer.
1) Count how many times each expression is present in the hierarchy.
1) For expression that are present many times, compute them only once
   per batch.

Author: akuzm

.unreleased/columnar-cache

@@ -0,0 +1 @@
+Implements: #9534 Speed up expression evaluation in the columnar pipeline by caching common subexpressions

src/compat/compat.h

@@ -304,6 +304,23 @@ RestrictSearchPath(void)
 					  0,
 					  false);
 }
+
+/*
+ * murmurhash64 was added to common/hashfn.h in PG17 (954e43564d9).
+ */
+static inline uint64
+murmurhash64(uint64 data)
+{
+	uint64 h = data;
+
+	h ^= h >> 33;
+	h *= 0xff51afd7ed558ccd;
+	h ^= h >> 33;
+	h *= 0xc4ceb9fe1a85ec53;
+	h ^= h >> 33;
+
+	return h;
+}
 #endif
 
 #if PG17_LT