[clang] [StaticAnalyzer] Fix InnerPointerBRVisitor to avoid repeated map scans (PR #185796)

[Gábor Horváth via cfe-commits]

Xazax-hun wrote:

The ctor is working with a different program state than `isSymbolTracked`. In case this change is not preserving the semantics of the original code I think the right fix would be to just remove the FIXME comment. This code path is only happening when we report bugs and this is not performance sensitive, we should not spend too much time optimizing it. 

https://github.com/llvm/llvm-project/pull/185796