simplify x^0 and x^1

[kroening]

The simplifier now rewrites two additional cases for power_exprt.

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• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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).
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[codecov]

Codecov Report

Attention: Patch coverage is 71.87500% with 9 lines in your changes missing coverage. Please review.

Project coverage is 78.77%. Comparing base (e8d3409) to head (8b0bef0).
Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/util/mathematical_expr.cpp	0.00%	4 Missing ⚠️
src/util/mathematical_expr.h	75.00%	2 Missing ⚠️
src/util/simplify_expr_int.cpp	77.77%	2 Missing ⚠️
unit/util/simplify_expr.cpp	90.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8509      +/-   ##
===========================================
+ Coverage    78.51%   78.77%   +0.25%     
===========================================
  Files         1728     1728              
  Lines       199553   198943     -610     
  Branches     18303    18302       -1     
===========================================
+ Hits        156687   156717      +30     
+ Misses       42866    42226     -640     

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