Mask generation optimization

[moh-eulith]

Abstract

I have a large contract compiled with --via-ir --optimize --optimize-runs 2
Looking at the generated opcodes I see the following mask generation patterns that can be optimized:

I've implemented a change to the constant optimizer for this here: develop...moh-eulith:solidity:mask_generation

May I create a PR from the above?

1. Mask generation
   15 instances of PUSH1 0x1 PUSH1 0x1 PUSH1 0x[0-9A-F]* SHL SUB NOT
   (sub case of the next one)
   -> PUSH0 NOT PUSH1 $bits SHL (saves 4 bytes)
   247 instances of PUSH1 0x1 PUSH1 0x1 PUSH1 0x[0-9A-F]* SHL SUB, saves 3*247 bytes
   PUSH1 0x1 PUSH1 0x1 PUSH1 0x40 SHL SUB : 8 bytes, 3 + 3 + 3 + 3 + 3 = 15 gas
   -> PUSH0 NOT PUSH1 0xC0 SHR: 5 bytes, 2 + 3 + 3 + 3 = 11 gas

2. Shifted mask generation
   53 instances of PUSH1 0x1 PUSH1 0x[0-9A-F]* SHL PUSH1 0x1 PUSH1 0x[0-9A-F]* SHL SUB: saves 3*53
   PUSH1 0x1 PUSH1 0x40 SHL PUSH1 0x1 PUSH1 0x80 SHL SUB: 11 opcodes
   -> PUSH0 NOT PUSH1 0xC0 SHR PUSH1 0x40 SHL: 8 opcodes

Motivation

Reduce both binary size and gas.

Specification

just an optimizer change, so no spec changes.

Backwards Compatibility

just an optimizer change, so backwards compatible.

[ekpyron]

Sorry again for the delay! We'd be happy to review a PR on this! We can also continue this in review, but be sure to note the coding style https://github.com/ethereum/solidity/blob/develop/CODING_STYLE.md (e.g. in the branch I see braces in single line if-statements that we usually skip); also the PR should get a Changelog entry (once #15927 is merged)

[moh-eulith]

Thank you for taking a look. PR submitted with the feedback. #15935