Automatically merged updates to draft EIP(s) 1884 (ethereum#2175)

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 - It only modifies existing Draft or Last Call EIP(s)
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EIPS/eip-1884.md

@@ -36,6 +36,7 @@ At block `N`,
 
 - The `SLOAD` (`0x54`) operation changes from `200` to `800` gas,
 - The `BALANCE` (`0x31`) operation changes from `400` to `700` gas,
+- The `EXTCODEHASH` (`0x3F`) operation changes from `400` to `700` gas,
 - A new opcode, `SELFBALANCE` is introduced at `0x46`. 
   - `SELFBALANCE` pops `0` arguments off the stack, 
   - `SELFBALANCE` pushes the `balance` of the current address to the stack,
@@ -91,6 +92,15 @@ opcodes: `EXTBALANCE(address)` and `SELFBALANCE`, and have two different prices.
   * As for why it is priced at `5` (`GasFastStep`) instead of `2` (`GasQuickStep`), like other similar operations: the EVM execution engine still needs a lookup into the (cached) trie, and `balance`, unlike `gasPrice` or `timeStamp`, is not constant during the execution, so it has a bit more inherent overhead. 
 
 
+### `EXTCODEHASH`
+
+`EXTCODEHASH` was introduced in Constantinople, with [EIP 1052](https://eips.ethereum.org/EIPS/eip-1052). It was priced at `400` with the reasoning:
+
+> The gas cost is the same as the gas cost for the `BALANCE` opcode because the execution of the `EXTCODEHASH` requires the same account lookup as in `BALANCE`.
+
+Ergo, if we increase `BALANCE`, we should also increase `EXTCODEHASH`
+
+
 ## Backwards Compatibility
 
 The changes require a hardfork. The changes have the following consequences: