perf(expr): optimize overlay with high for input

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dev=> \timing on
Timing is on.
dev=> SELECT overlay('123' placing '12345' from 1123456 for 1123456); 
 ?column? 
----------
 12312345
(1 row)

Time: 47.893 ms
dev=> SELECT overlay('123' placing '12345' from 1123456789 for 1123456789); 
 ?column? 
----------
 12312345
(1 row)

Time: 10451.279 ms (00:10.451)
dev=> SELECT overlay('123' placing '12345' from 1123456789 for 11234); 
 ?column? 
----------
 12312345
(1 row)

Time: 10423.252 ms (00:10.423)
dev=> SELECT overlay('123' placing '12345' from 11234 for 1123456789); 
 ?column? 
----------
 12312345
(1 row)

Time: 4.833 ms