Allow use of `consts` in place of immediates in asm blocks

## Motivation
https://github.com/FuelLabs/sway-lib-std/pull/46
We should be able to directly use the `const` in the asm block if it is small enough to fit in that immediate size.

## Summary

Currently, immediates are size-checked in the compiler. If an opcode takes a 12-bit immediate, the size of that immediate is checked at compile time. Since we also know the value of `const`s at compile time, we should be able to directly allow the use of `const`s inside of `asm` blocks instead of passing them in. This will require a modification to the grammar and then size checking the const.

Start by reading the linked sway-lib-std pull request and the compiler's asm grammar and immediate size-checking paths. Trace how compile-time const values are represented in asm, then verify that a const is accepted only when its value fits the opcode's immediate size.

Written by the indexing model from the issue text.