[x64] Make r12 allocatable.

src/x64/regexp-macro-assembler-x64.cc

Index: src/x64/regexp-macro-assembler-x64.cc
diff --git a/src/x64/regexp-macro-assembler-x64.cc b/src/x64/regexp-macro-assembler-x64.cc
index 1818dbb72c7444658666bb20640bbea6bda6d8a4..050c27302d378b9f4cb8c20c5b51c428689dce88 100644
--- a/src/x64/regexp-macro-assembler-x64.cc
+++ b/src/x64/regexp-macro-assembler-x64.cc
@@ -43,10 +43,9 @@ namespace internal {
  *
  * The registers rax, rbx, r9 and r11 are free to use for computations.
  * If changed to use r12+, they should be saved as callee-save registers.
- * The macro assembler special registers r12 and r13 (kSmiConstantRegister,
- * kRootRegister) aren't special during execution of RegExp code (they don't
- * hold the values assumed when creating JS code), so no Smi or Root related
- * macro operations can be used.
+ * The macro assembler special register r13 (kRootRegister) isn't special
+ * during execution of RegExp code (it doesn't hold the value assumed when
+ * creating JS code), so Root related macro operations can be used.
  *
  * Each call to a C++ method should retain these registers.
  *