[move-function] Move diagnostics in front of the flag.

This is safe to do since:

1. _move is already an always emit into client transparent function, so no ABI
   impact comes from it.

2. _move is underscored so it is a non-stable stdlib API meaning we aren't
   providing any guarantees here.

3. Code that doesn't use _move will not be effected by this so there isn't an
   impact on other code.

Author: gottesmm

lib/SILOptimizer/Mandatory/MoveFunctionCanonicalization.cpp

@@ -272,11 +272,6 @@ class MoveFunctionCanonicalization : public SILFunctionTransform {
   void run() override {
     auto *fn = getFunction();
 
-    // If we do not have experimental move only enabled, do not emit
-    // diagnostics.
-    if (!astContext.LangOpts.EnableExperimentalMoveOnly)
-      return;
-
     // Don't rerun diagnostics on deserialized functions.
     if (getFunction()->wasDeserializedCanonical())
       return;

lib/SILOptimizer/Mandatory/MoveKillsCopyableAddressesChecker.cpp

@@ -961,11 +961,6 @@ class MoveKillsCopyableAddressesCheckerPass : public SILFunctionTransform {
     auto *fn = getFunction();
     auto &astContext = fn->getASTContext();
 
-    // If we do not have experimental move only enabled, do not emit
-    // diagnostics.
-    if (!astContext.LangOpts.EnableExperimentalMoveOnly)
-      return;
-
     // Don't rerun diagnostics on deserialized functions.
     if (getFunction()->wasDeserializedCanonical())
       return;

lib/SILOptimizer/Mandatory/MoveKillsCopyableValuesChecker.cpp

@@ -431,11 +431,6 @@ class MoveKillsCopyableValuesCheckerPass : public SILFunctionTransform {
     auto *fn = getFunction();
     auto &astContext = fn->getASTContext();
 
-    // If we do not have experimental move only enabled, do not emit
-    // diagnostics.
-    if (!astContext.LangOpts.EnableExperimentalMoveOnly)
-      return;
-
     // Don't rerun diagnostics on deserialized functions.
     if (getFunction()->wasDeserializedCanonical())
       return;