Add --no-simplify-phi option

[peterschrammel]

Allow disabling the simplification of phi functions as this is very expensive operation.

Using this option is not recommended in general, but in certain cases it has been observed to cut symex time in half without noticeable influence on SAT solving time.

Also removes the orphaned --no-simplify-if option in a separate commit.

• 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.
• [no] 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).
• [no] My commit message includes data points confirming performance improvements (if claimed).
• [almost] My PR is restricted to a single feature or bugfix.
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[tautschnig]

I wonder whether the removal of no-simplify-if should be a PR of its own and be tagged "Version 6" -- or at least that's what I thought when doing #6919 (well, I should have created that PR back then...).

[tautschnig]

Also, can those costly benchmarks be named/shared so that we can consider ways to improve the simplifier performance instead? Edit: I do wonder whether #6118 might make some difference in terms of performance.