[arm64] Remove duplicated register logic

Instead of having both a templatized and a parameter-based method, just
have the parameter-based one, but make it constexpr. DCHECKs in these
constexpr methods need to be guarded because GCC 5 does not support them
yet.

R=thibaudm@chromium.org

Bug: v8:10021
Change-Id: If362de42f1a4c2cda4435fce63beb9244795d008
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1986001Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65611}

src/codegen/arm64/register-arm64.h

@@ -108,14 +108,10 @@ class CPURegister : public RegisterBase<CPURegister, kRegAfterLast> {
     return CPURegister{kCode_no_reg, 0, kNoRegister};
   }
 
-  template <int code, int size, RegisterType type>
-  static constexpr CPURegister Create() {
-    static_assert(IsValid(code, size, type), "Cannot create invalid registers");
-    return CPURegister{code, size, type};
-  }
-
-  static CPURegister Create(int code, int size, RegisterType type) {
+  static constexpr CPURegister Create(int code, int size, RegisterType type) {
+#if V8_HAS_CXX14_CONSTEXPR
     DCHECK(IsValid(code, size, type));
+#endif
     return CPURegister{code, size, type};
   }
 
@@ -242,29 +238,18 @@ class Register : public CPURegister {
  public:
   static constexpr Register no_reg() { return Register(CPURegister::no_reg()); }
 
-  template <int code, int size>
-  static constexpr Register Create() {
-    return Register(CPURegister::Create<code, size, CPURegister::kRegister>());
-  }
-
-  static Register Create(int code, int size) {
+  static constexpr Register Create(int code, int size) {
     return Register(CPURegister::Create(code, size, CPURegister::kRegister));
   }
 
   static Register XRegFromCode(unsigned code);
   static Register WRegFromCode(unsigned code);
 
-  static Register from_code(int code) {
+  static constexpr Register from_code(int code) {
     // Always return an X register.
     return Register::Create(code, kXRegSizeInBits);
   }
 
-  template <int code>
-  static Register from_code() {
-    // Always return an X register.
-    return Register::Create<code, kXRegSizeInBits>();
-  }
-
   static const char* GetSpecialRegisterName(int code) {
     return (code == kSPRegInternalCode) ? "sp" : "UNKNOWN";
   }
@@ -335,14 +320,10 @@ class VRegister : public CPURegister {
     return VRegister(CPURegister::no_reg(), 0);
   }
 
-  template <int code, int size, int lane_count = 1>
-  static constexpr VRegister Create() {
-    static_assert(IsValidLaneCount(lane_count), "Invalid lane count");
-    return VRegister(CPURegister::Create<code, size, kVRegister>(), lane_count);
-  }
-
-  static VRegister Create(int code, int size, int lane_count = 1) {
+  static constexpr VRegister Create(int code, int size, int lane_count = 1) {
+#if V8_HAS_CXX14_CONSTEXPR
     DCHECK(IsValidLaneCount(lane_count));
+#endif
     return VRegister(CPURegister::Create(code, size, CPURegister::kVRegister),
                      lane_count);
   }
@@ -459,7 +440,7 @@ constexpr Register no_reg = NoReg;
 constexpr VRegister no_dreg = NoVReg;
 
 #define DEFINE_REGISTER(register_class, name, ...) \
-  constexpr register_class name = register_class::Create<__VA_ARGS__>()
+  constexpr register_class name = register_class::Create(__VA_ARGS__)
 #define ALIAS_REGISTER(register_class, alias, name) \
   constexpr register_class alias = name