[DebugInfo] Salvage more in -O builds

Specifically, improved debug info retention in:
* tryReplaceRedundantInstructionPair,
* splitAggregateLoad,
* TempLValueElimination,
* Mem2Reg,
* ConstantFolding.

The changes to Mem2Reg allow debug info to be retained in the case tested by
self-nostorage.swift in -O builds, so we have just enabled -O in that file
instead of writing a new test for it.

We attempted to add a case to salvageDebugInfo for unchecked_enum_data, but it
caused crashes in Linux CI that we were not able to reproduce.

SwiftCompilerSources/Sources/SIL/Instruction.swift

@@ -102,6 +102,13 @@ public class Instruction : CustomStringConvertible, Hashable {
     context.notifyInstructionsChanged()
   }
 
+  /// Transfer debug info associated with (the result of) this instruction to a
+  /// new `debug_value` instruction before this instruction is deleted.
+  public final func salvageDebugInfo(_ context: some MutatingContext) {
+    BridgedContext.salvageDebugInfo(self.bridged)
+    context.notifyInstructionsChanged()
+  }
+
   public var mayTrap: Bool { false }
 
   final public var mayHaveSideEffects: Bool {