[NFC] Add AccessBase.address

LifetimeDependence uses the address of the AccessBase to compute liveness. It is
more convenient to switch over all AccessBase kinds here rather than force all
clients to do it.

Author: atrick

SwiftCompilerSources/Sources/SIL/Utilities/AccessUtils.swift

@@ -114,6 +114,22 @@ public enum AccessBase : CustomStringConvertible, Hashable {
     }
   }
 
+  /// Return 'nil' for global varabiables and unidentified addresses.
+  public var address: Value? {
+    switch self {
+      case .global, .unidentified: return nil
+      case .box(let pbi):        return pbi
+      case .stack(let asi):      return asi
+      case .class(let rea):      return rea
+      case .tail(let rta):       return rta
+      case .argument(let arg):   return arg
+      case .yield(let result):   return result
+      case .storeBorrow(let sb): return sb
+      case .pointer(let p):      return p
+      case .index(let ia):       return ia
+    }
+  }
+
   public var description: String {
     switch self {
       case .unidentified:      return "?"