X86_64: Pass 'hidden method index' in EAX

Method* is in EDI, and EAX isn't an argument register, so EAX is free
to hold the hidden method index.

Change-Id: I793a54d00a4593e140f97144419d849b53bfdf44
Signed-off-by: Mark Mendell <mark.p.mendell@intel.com>
diff --git a/compiler/dex/quick/x86/target_x86.cc b/compiler/dex/quick/x86/target_x86.cc
index 1ac15a2..82f7fad 100644
--- a/compiler/dex/quick/x86/target_x86.cc
+++ b/compiler/dex/quick/x86/target_x86.cc
@@ -216,7 +216,7 @@
     case kRet1: res_reg = rs_rX86_RET1; break;
     case kInvokeTgt: res_reg = rs_rX86_INVOKE_TGT; break;
     case kHiddenArg: res_reg = rs_rAX; break;
-    case kHiddenFpArg: res_reg = rs_fr0; break;
+    case kHiddenFpArg: DCHECK(!Gen64Bit()); res_reg = rs_fr0; break;
     case kCount: res_reg = rs_rX86_COUNT; break;
     default: res_reg = RegStorage::InvalidReg();
   }