MIPS64: Fix declaration for bit() function.

From declarations of bit() member functions (for structures Register,
FPURegister and FPUControlRegister) are removed constexpr specificators.

Build V8 will fail if function bit() is declared as a constant
expression.

TEST=
BUG=

Review-Url: https://codereview.chromium.org/2737143002
Cr-Commit-Position: refs/heads/master@{#43674}

src/mips64/assembler-mips64.h

@@ -132,7 +132,10 @@ struct Register {
     DCHECK(is_valid());
     return reg_code;
   }
-  constexpr int bit() const { return DCHECK(is_valid()), 1 << reg_code; }
+  int bit() const {
+    DCHECK(is_valid());
+    return 1 << reg_code;
+  }
 
   // Unfortunately we can't make this private in a struct.
   int reg_code;
@@ -196,7 +199,11 @@ struct FPURegister {
     DCHECK(is_valid());
     return reg_code;
   }
-  constexpr int bit() const { return DCHECK(is_valid()), 1 << reg_code; }
+
+  int bit() const {
+    DCHECK(is_valid());
+    return 1 << reg_code;
+  }
 
   static FPURegister from_code(int code) {
     FPURegister r = {code};
@@ -287,7 +294,10 @@ struct FPUControlRegister {
     DCHECK(is_valid());
     return reg_code;
   }
-  constexpr int bit() const { return DCHECK(is_valid()), 1 << reg_code; }
+  int bit() const {
+    DCHECK(is_valid());
+    return 1 << reg_code;
+  }
   void setcode(int f) {
     reg_code = f;
     DCHECK(is_valid());