[DebugInfo] Fix PhiExpansion losing debug info

When replacing a struct with its field, it now emits a debug_value
with a fragment, instead of dropping the debug_value.

Author: Snowy1803

lib/SILOptimizer/Transforms/PhiArgumentOptimizations.cpp

@@ -439,6 +439,13 @@ bool PhiExpansionPass::optimizeArg(SILPhiArgument *initialArg) {
     }
 
     for (DebugValueInst *dvi : debugValueUsers) {
+      // Recreate the debug_value with a fragment.
+      SILBuilder B(dvi, dvi->getDebugScope());
+      SILDebugVariable var = *dvi->getVarInfo();
+      if (!var.Type)
+        var.Type = initialArg->getType();
+      var.DIExpr.append(SILDebugInfoExpression::createFragment(field));
+      B.createDebugValue(dvi->getLoc(), dvi->getOperand(), var);
       dvi->eraseFromParent();
     }
     for (StructExtractInst *sei : structExtractUsers) {