[instcombine] Adjust style of MemIntrinsic code to be more idiomatic [nfc] (#138715)

Use an existing helper function. Remove the use of a local Changed
variable which doesn't seem to interact with surrounding transforms in
any meaningful way. (Both memcpy and memmove are MemTransfer
instructions, so switching from one to the other doesn't change
results.)

Posted for review mostly for a sanity check that I'm not missing
something with the logic around the Change flag.

Author: preames

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

@@ -1671,38 +1671,15 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
   // Intrinsics cannot occur in an invoke or a callbr, so handle them here
   // instead of in visitCallBase.
   if (auto *MI = dyn_cast<AnyMemIntrinsic>(II)) {
-    bool Changed = false;
-
     // memmove/cpy/set of zero bytes is a noop.
     if (Constant *NumBytes = dyn_cast<Constant>(MI->getLength())) {
       if (NumBytes->isNullValue())
         return eraseInstFromFunction(CI);
     }
 
     // No other transformations apply to volatile transfers.
-    if (auto *M = dyn_cast<MemIntrinsic>(MI))
-      if (M->isVolatile())
-        return nullptr;
-
-    // If we have a memmove and the source operation is a constant global,
-    // then the source and dest pointers can't alias, so we can change this
-    // into a call to memcpy.
-    if (auto *MMI = dyn_cast<AnyMemMoveInst>(MI)) {
-      if (GlobalVariable *GVSrc = dyn_cast<GlobalVariable>(MMI->getSource()))
-        if (GVSrc->isConstant()) {
-          Module *M = CI.getModule();
-          Intrinsic::ID MemCpyID =
-              isa<AtomicMemMoveInst>(MMI)
-                  ? Intrinsic::memcpy_element_unordered_atomic
-                  : Intrinsic::memcpy;
-          Type *Tys[3] = { CI.getArgOperand(0)->getType(),
-                           CI.getArgOperand(1)->getType(),
-                           CI.getArgOperand(2)->getType() };
-          CI.setCalledFunction(
-              Intrinsic::getOrInsertDeclaration(M, MemCpyID, Tys));
-          Changed = true;
-        }
-    }
+    if (MI->isVolatile())
+      return nullptr;
 
     if (AnyMemTransferInst *MTI = dyn_cast<AnyMemTransferInst>(MI)) {
       // memmove(x,x,size) -> noop.
@@ -1734,7 +1711,25 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
       return eraseInstFromFunction(CI);
     }
 
-    if (Changed) return II;
+    // If we have a memmove and the source operation is a constant global,
+    // then the source and dest pointers can't alias, so we can change this
+    // into a call to memcpy.
+    if (auto *MMI = dyn_cast<AnyMemMoveInst>(MI)) {
+      if (GlobalVariable *GVSrc = dyn_cast<GlobalVariable>(MMI->getSource()))
+        if (GVSrc->isConstant()) {
+          Module *M = CI.getModule();
+          Intrinsic::ID MemCpyID =
+              isa<AtomicMemMoveInst>(MMI)
+                  ? Intrinsic::memcpy_element_unordered_atomic
+                  : Intrinsic::memcpy;
+          Type *Tys[3] = { CI.getArgOperand(0)->getType(),
+                           CI.getArgOperand(1)->getType(),
+                           CI.getArgOperand(2)->getType() };
+          CI.setCalledFunction(
+              Intrinsic::getOrInsertDeclaration(M, MemCpyID, Tys));
+          return II;
+        }
+    }
   }
 
   // For fixed width vector result intrinsics, use the generic demanded vector