[MachineLoopInfo] Fix getLoopID to handle multi latches.

This patch also fixed CodegenPrepare to reserve loop metadata
when merging blocks.

This fixes issue #102632

Author: FreddyLeaf

llvm/lib/CodeGen/CodeGenPrepare.cpp

@@ -1183,6 +1183,12 @@ void CodeGenPrepare::eliminateMostlyEmptyBlock(BasicBlock *BB) {
     }
   }
 
+  // Reserve loop Metadata.
+  if (BI->hasMetadata(LLVMContext::MD_loop)) {
+    for (auto *Pred : predecessors(BB))
+      Pred->getTerminator()->copyMetadata(*BI, LLVMContext::MD_loop);
+  }
+
   // The PHIs are now updated, change everything that refers to BB to use
   // DestBB and remove BB.
   BB->replaceAllUsesWith(DestBB);

llvm/lib/CodeGen/MachineLoopInfo.cpp

@@ -211,8 +211,6 @@ MDNode *MachineLoop::getLoopID() const {
             break;
           }
         }
-        if (!MD)
-          return nullptr;
         if (!LoopID)
           LoopID = MD;
         else if (MD != LoopID)

llvm/test/CodeGen/X86/code-align-loops.ll

@@ -111,7 +111,7 @@ for.body5:                                        ; preds = %for.body, %for.body
 ;     }
 ; }
 ; CHECK-LABEL: test3_multilatch:
-; ALIGN: .p2align 4, 0x90
+; ALIGN: .p2align 6, 0x90
 ; ALIGN-NEXT: .LBB2_1: # %while.cond
 define dso_local i32 @test3_multilatch() #0 {
 entry: