[Debuginfo][LSR] Add salvaging variadic dbg.value intrinsics [2/2]

Reland 3f2b76e with the test corrected
to require x86-registered-target.

Differential Revision: https://reviews.llvm.org/D120169

Author: Chris Jackson

llvm/include/llvm/Analysis/ScalarEvolution.h

@@ -545,6 +545,10 @@ class ScalarEvolution {
   /// Return true if the SCEV expression contains an undef value.
   bool containsUndefs(const SCEV *S) const;
 
+  /// Return true if the SCEV expression contains a Value that has been
+  /// optimised out and is now a nullptr.
+  bool containsErasedValue(const SCEV *S) const;
+
   /// Return a SCEV expression for the full generality of the specified
   /// expression.
   const SCEV *getSCEV(Value *V);

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -12688,6 +12688,15 @@ bool ScalarEvolution::containsUndefs(const SCEV *S) const {
   });
 }
 
+// Return true when S contains a value that is a nullptr.
+bool ScalarEvolution::containsErasedValue(const SCEV *S) const {
+  return SCEVExprContains(S, [](const SCEV *S) {
+    if (const auto *SU = dyn_cast<SCEVUnknown>(S))
+      return SU->getValue() == nullptr;
+    return false;
+  });
+}
+
 /// Return the size of an element read or written by Inst.
 const SCEV *ScalarEvolution::getElementSize(Instruction *Inst) {
   Type *Ty;