GROOVY-11754: STC: prevent cast exception under imbalanced instanceof

[eric-milles]

Changes to StaticTypesTypeChooser in Groovy 5 caused a cast exception at runtime for statically-compiled imbalanced-instanceof scenarios like if (cn instanceof InnerClassNode || cn.isEnum()). This traces back to the temp types storage in STC. For the statement above, only InnerClassNode is saved as a temp type. When the temp type is computed for "cn.isEnum()", InnerClassNode is the result.

A small part of the GROOVY-7971 solution is to mix in the original variable type in a scenario such as this -- to express the optionality of instance check(s). I back-ported just this part of the larger solution. This stops the cast exception for the GROOVY-11754 scenario. This is a pre-condition for introducing the GROOVY-11769 StaticTypesTypeChooser change (see #2311) into Groovy 4.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.71429% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.1339%. Comparing base (7d36495) to head (781f35e).

Files with missing lines	Patch %	Lines
...roovy/transform/stc/StaticTypeCheckingVisitor.java	85.7143%	0 Missing and 5 partials ⚠️

Additional details and impacted files

@@                  Coverage Diff                   @@
##             GROOVY_5_0_X      #2397        +/-   ##
======================================================
+ Coverage         67.1286%   67.1339%   +0.0053%     
- Complexity          29422      29430         +8     
======================================================
  Files                1382       1382                
  Lines              116758     116789        +31     
  Branches            20462      20471         +9     
======================================================
+ Hits                78378      78405        +27     
- Misses              31902      31903         +1     
- Partials             6478       6481         +3     

Files with missing lines	Coverage Δ
...roovy/transform/stc/StaticTypeCheckingVisitor.java	87.7494% <85.7143%> (-0.0057%)	⬇️

... and 4 files with indirect coverage changes

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