Cache current thread in a reserved register and use it in LoadIsolate

runtime/vm/constants_x64.h

Index: runtime/vm/constants_x64.h
diff --git a/runtime/vm/constants_x64.h b/runtime/vm/constants_x64.h
index d42687e2332f4001a9dd6cd80959cc89b40b0d0d..fdded5f7cfb5ed6107e74ac820b71fb98a5ea772 100644
--- a/runtime/vm/constants_x64.h
+++ b/runtime/vm/constants_x64.h
@@ -86,13 +86,14 @@ enum RexBits {
 // Register aliases.
 const Register TMP = R11;  // Used as scratch register by the assembler.
 const Register TMP2 = kNoRegister;  // No second assembler scratch register.
-const Register CTX = R14;  // Location of current context at method entry.
+const Register CTX = R9;  // Location of current context at method entry.
 // Caches object pool pointer in generated code.
 const Register PP = R15;
 const Register SPREG = RSP;  // Stack pointer register.
 const Register FPREG = RBP;  // Frame pointer register.
 const Register ICREG = RBX;  // IC data register.
 const Register ARGS_DESC_REG = R10;  // Arguments descriptor register.
+const Register THR = R14;  // Caches current thread in generated code.
 
 // Exception object is passed in this register to the catch handlers when an
 // exception is thrown.