[instcombine] Adjust style of MemIntrinsic code to be more idiomatic [nfc]

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.)

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