Simplify overflow-mult when one operand is 1

[tautschnig]

1 (one) is a neutral element for multiplication.

Depending on the order of reviews/approvals, this PR or #6633 will require merge conflict resolution: #6633 introduces additional checks, some of which can be simplified away via the changes in this PR.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6634 (c2cbaad) into develop (a638644) will increase coverage by 0.00%.
The diff coverage is 95.25%.

@@           Coverage Diff            @@
##           develop    #6634   +/-   ##
========================================
  Coverage    76.65%   76.65%           
========================================
  Files         1580     1579    -1     
  Lines       181560   181612   +52     
========================================
+ Hits        139168   139220   +52     
  Misses       42392    42392           

Impacted Files	Coverage Δ
src/goto-instrument/contracts/contracts.h	100.00% <ø> (ø)
src/goto-programs/restrict_function_pointers.h	100.00% <ø> (ø)
src/goto-instrument/contracts/utils.cpp	85.59% <80.55%> (ø)
src/goto-programs/restrict_function_pointers.cpp	80.27% <89.58%> (ø)
...trument/contracts/havoc_assigns_clause_targets.cpp	95.55% <93.33%> (ø)
...o-instrument/contracts/instrument_spec_assigns.cpp	95.35% <95.35%> (ø)
src/ansi-c/c_typecheck_code.cpp	79.92% <100.00%> (ø)
src/goto-instrument/contracts/contracts.cpp	90.75% <100.00%> (ø)
...nstrument/contracts/havoc_assigns_clause_targets.h	100.00% <100.00%> (ø)
...oto-instrument/contracts/instrument_spec_assigns.h	100.00% <100.00%> (ø)
... and 17 more

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