[ARM64] [turbofan] Use 'mvn' rather than an equivalent 'orn'.

`mvn xd, xn` is an architectural alias for `orn xd, xzr, xm`, so this
doesn't change code generation at all, but it's tidier and it addresses
a TODO.

BUG=

Review URL: https://codereview.chromium.org/1151483002

Cr-Commit-Position: refs/heads/master@{#28507}

src/compiler/arm64/code-generator-arm64.cc

@@ -558,12 +558,11 @@ void CodeGenerator::AssembleArchInstruction(Instruction* instr) {
               i.InputRegister32(0));
       break;
     }
-    // TODO(dcarney): use mvn instr??
     case kArm64Not:
-      __ Orn(i.OutputRegister(), xzr, i.InputOperand(0));
+      __ Mvn(i.OutputRegister(), i.InputOperand(0));
       break;
     case kArm64Not32:
-      __ Orn(i.OutputRegister32(), wzr, i.InputOperand32(0));
+      __ Mvn(i.OutputRegister32(), i.InputOperand32(0));
       break;
     case kArm64Neg:
       __ Neg(i.OutputRegister(), i.InputOperand(0));