fix(ssa): Slice mergers with multiple ifs

[vezenovm]

Description

Problem*

Resolves

No issue as this was found and quickly patched as part of #374

Summary*

The handling of get_slice_length was incorrect in that for some instruction cases it used the slice capacity (the total number of slice elements in an array value) and in some cases it used the slice length prefix. During slice merger we should only ever use the capacity. The length is to be used only for checking against user out of bounds accesses. This was not a problem in the existing tests, but when trying to do a push back outside of an if statement where there was a slice merger we get mismatched slice values as the length is not correct.

Documentation

• This PR requires documentation updates when merged.

  • I will submit a noir-lang/docs PR.
  • I will request for and support Dev Rel's help in documenting this PR.

Additional Context

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[vezenovm]

I have made #2599 as even though we can merge slices with multiple if statements, currently there is a bug if there is not an explicit else case.