[Turbofan] Reduce register allocation work when we can.

src/machine-type.h

Index: src/machine-type.h
diff --git a/src/machine-type.h b/src/machine-type.h
index 69cb6d79fac6062e6771d68624ba0463385a5215..18f6752b77605b08368da81062085d31974ceafa 100644
--- a/src/machine-type.h
+++ b/src/machine-type.h
@@ -29,9 +29,14 @@ enum class MachineRepresentation : uint8_t {
   kFloat32,
   kFloat64,
   kSimd128,
-  kFirstFPRepresentation = kFloat32
+  kFirstFPRepresentation = kFloat32,
+  kLastRepresentation = kSimd128
 };
 
+static_assert(static_cast<int>(MachineRepresentation::kLastRepresentation) <
+                  kIntSize * kBitsPerByte,
+              "Bit masks of MachineRepresentation should fit in an int");
+
 const char* MachineReprToString(MachineRepresentation);
 
 enum class MachineSemantic : uint8_t {