[PATCH] Replace a long chain of if's in eval_const_expressions_mutator by a switch()

Hi,

While playing around with writing a newer allocator last week (using a 
somewhat slab allocator alike architecture) I found 
eval_const_expressions_mutator to be surprisingly high in profiles.

For unknown reasons the function used non chained ifs for every handled
nodeType.

Replacing the if chain with if; else if; ... resulted in a small
speedup. Replacing it with a switch() in a bigger one.

When testing with a statement containing a longer VALUES statement:
pgbench -M prepared -f stmt -T 10:
orig: 10015.28
if: 10075.48
switch: 10246.52

Thats a 2.3% increase over several runs. Other statements yielded smaller but 
all positive results.

As far as I can see there currently is no point for writing the function as it 
is right now. Am I missing something?

Andres