Mask generation optimization

## 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: https://github.com/ethereum/solidity/compare/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.

Start with the constant optimizer and compare its behavior with the referenced mask_generation branch. Verify that the listed mask and shifted-mask opcode patterns are replaced while preserving behavior, and check the stated byte-size and gas reductions in generated opcodes.

Written by the indexing model from the issue text.