MMIO rewriting must maintain type consistency

[tautschnig]

MMIO read functions may (have to) return values of varying types.
Expression replacement must maintain, via type casts, consistency with
whatever the type of the original expression was.

• 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 #6683 (871d3a5) into develop (222d266) will increase coverage by 0.00%.
The diff coverage is 97.08%.

@@           Coverage Diff            @@
##           develop    #6683   +/-   ##
========================================
  Coverage    76.82%   76.82%           
========================================
  Files         1582     1582           
  Lines       182767   182770    +3     
========================================
+ Hits        140403   140411    +8     
+ Misses       42364    42359    -5     

Impacted Files	Coverage Δ
src/goto-programs/string_abstraction.h	100.00% <ø> (ø)
unit/util/piped_process.cpp	31.29% <33.33%> (ø)
src/ansi-c/c_typecheck_expr.cpp	76.45% <50.00%> (ø)
src/goto-programs/string_abstraction.cpp	92.46% <90.90%> (ø)
src/goto-programs/mm_io.cpp	100.00% <100.00%> (ø)
...solvers/smt2_incremental/smt_bit_vector_theory.cpp	100.00% <100.00%> (ø)
...rc/solvers/smt2_incremental/smt_solver_process.cpp	68.57% <100.00%> (ø)
src/util/piped_process.cpp	81.92% <100.00%> (ø)
...solvers/smt2_incremental/smt_bit_vector_theory.cpp	100.00% <100.00%> (ø)
... and 7 more

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