Enable LoopRotate on header blocks with forwarding instructions

We now have support for guaranteed forwarding phis.

Fixes rdar://86210259

Author: meg-gupta

lib/SILOptimizer/LoopTransforms/LoopRotate.cpp

@@ -79,11 +79,6 @@ canDuplicateOrMoveToPreheader(SILLoop *loop, SILBasicBlock *preheader,
     if (isa<DeallocStackInst>(inst)) {
       return false;
     }
-    OwnershipForwardingMixin *ofm = nullptr;
-    if ((ofm = OwnershipForwardingMixin::get(inst)) &&
-        ofm->getForwardingOwnershipKind() == OwnershipKind::Guaranteed) {
-      return false;
-    }
     if (isa<FunctionRefInst>(inst)) {
       moves.push_back(inst);
       invariants.insert(inst);

test/SILOptimizer/looprotate_nontrivial_ossa.sil

@@ -203,7 +203,7 @@ exit:
 // A guaranteed value whose ownership has been forwarded must not be reborrowed. 
 //
 // CHECK-LABEL: sil [ossa] @forwarded_borrow_cant_be_reborrowed : $@convention(thin) (@owned BoxStruct) -> () {
-// CHECK-NOT:   {{bb[0-9]+}}({{%[^,]+}} : @guaranteed $BoxStruct, {{%[^,]+}} : @guaranteed $Klass):
+// CHECK:   {{bb[0-9]+}}({{%[^,]+}} : @guaranteed $BoxStruct, {{%[^,]+}} : @guaranteed $Klass):
 // CHECK-LABEL: } // end sil function 'forwarded_borrow_cant_be_reborrowed'
 sil [ossa] @forwarded_borrow_cant_be_reborrowed : $@convention(thin) (@owned BoxStruct) -> () {
 bb0(%0 : @owned $BoxStruct):