NFC: Comment, refactor, and test hlsl::RemoveUnstructuredLoopExits (microsoft#6655)

Add a pass to run hlsl::RemoveUnstructuredLoopExits in isolation

  Example: opt -dxil-r-u-l-e a.ll -S

Add some basic tests.

No functional change to the pass itself.

Author: dneto0

include/llvm/InitializePasses.h

@@ -272,6 +272,7 @@ void initializeDxilReinsertNopsPass(PassRegistry&);
 void initializeDxilFinalizePreservesPass(PassRegistry&);
 void initializeDxilPreserveToSelectPass(PassRegistry&);
 void initializeDxilRemoveDeadBlocksPass(PassRegistry&);
+void initializeDxilRemoveUnstructuredLoopExitsPass(PassRegistry &);
 void initializeDxilRewriteOutputArgDebugInfoPass(PassRegistry&);
 // HLSL Change Ends
 void initializeScalarEvolutionAliasAnalysisPass(PassRegistry&);

include/llvm/Transforms/Scalar.h

@@ -153,6 +153,9 @@ void initializeDxilPreserveToSelectPass(PassRegistry&);
 Pass *createDxilRemoveDeadBlocksPass();
 void initializeDxilRemoveDeadBlocksPass(PassRegistry&);
 
+Pass *createDxilRemoveUnstructuredLoopExitsPass();
+void initializeDxilRemoveUnstructuredLoopExitsPass(PassRegistry &);
+
 void initializeDxilRewriteOutputArgDebugInfoPass(PassRegistry&);
 Pass *createDxilRewriteOutputArgDebugInfoPass();