[SwiftCompilerSources] Handle .anyValueFields use-def walks through enums

The walker was not treating an EnumInst with zero payload, such as `Optional.none` as a root.
It seems the best way to fix that is to implement the handling of .anyValueFields for enums, as
they're documented in a comment to mean "follow anything", unlike .enumCase which expects
to find a specific case (though perhaps if it matches and there's no payload, it should still be a root?)

Author: kavon

SwiftCompilerSources/Sources/Optimizer/Utilities/WalkUtils.swift

@@ -631,6 +631,15 @@ extension ValueUseDefWalker {
       if let path = path.popIfMatches(.enumCase, index: e.caseIndex),
          let payload = e.payload {
         return walkUp(value: payload, path: path)
+
+      } else if path.popIfMatches(.anyValueFields, index: nil) != nil {
+        if let payload = e.payload {
+          return walkUp(value: payload, path: path)
+        } else {
+          // without a payload, this enum is itself a definition root.
+          return rootDef(value: e, path: path)
+        }
+
       } else {
         return unmatchedPath(value: e, path: path)
       }