fix StmtGraph.replaceNode(sameStmt, sameStmt) and Stmt.withNewUse()

[swissiety]

• consistently return the former stmt instead of null
• MutableBlockStmtGraph.replaceNode() failed maintaining index datastructures when replacing a stmt with itself

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 69.95%. Comparing base (e63dc81) to head (1d68837).

Files	Patch %	Lines
.../java/sootup/core/graph/MutableBlockStmtGraph.java	0.00%	1 Missing and 1 partial ⚠️
...tup/core/jimple/visitor/ReplaceUseStmtVisitor.java	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #885      +/-   ##
=============================================
+ Coverage      69.88%   69.95%   +0.06%     
- Complexity      4039     4042       +3     
=============================================
  Files            310      310              
  Lines          15248    15245       -3     
  Branches        2605     2602       -3     
=============================================
+ Hits           10656    10664       +8     
+ Misses          3735     3727       -8     
+ Partials         857      854       -3     

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