Error for uninitialized register on asm blocks

[xunilrj]

Description

This PR closes #4997. The problem here is that "asm blocks registers" without initializers are considered uninitialized. There is no fallback to "capture" an available variable. This could be easily done, but that would leave us with no obvious syntax for unitialized registers.

With this in mind this PR solves the compiler bug with two diagnostics:

1 - uninitialized registers cannot be read before they are written. This generates an error:

 5 |     let _ = asm(r1) {
   |                 ^^ Unitialized register is being read before being written
 6 |         r1: u64
 7 |     };

2 - a warning when a unitialized register shadows an available variable with the same name. This is almost for sure a problem, and it is better the name the registe with something else and avoid all the confusion.

 27 |     let r5 = 0;
 28 |     asm(r5) {};
    |         -- This unitialized register is shadowing a variable, you probably meant to also initialize it like "r5: r5".
 29 | 
 30 |     0

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.