Assigns clause should only check assignments over variables from the write set

[feliperodri]

CBMC version: develop branch.
Operating system: N/A.
Exact command line resulting in the issue: N/A.
What behaviour did you expect: We only want to check assignments over variables from the write set. Local variables should be ignored.
What happened instead: When using assigns clause, CBMC checks for all assignments, even if they are over local variables.

[SaswatPadhi]

Context: #6108 (comment)