x86_64: Clean-up after cmp-long fix

The patch adresses the coments from review done by Ian Rogers.
Clean-up of assembler.

Change-Id: I9dbb350dfc6645f8a63d624b2b785233529459a9
Signed-off-by: Serguei Katkov <serguei.i.katkov@intel.com>
diff --git a/compiler/dex/quick/x86/x86_lir.h b/compiler/dex/quick/x86/x86_lir.h
index e271e9d..05c4b3d 100644
--- a/compiler/dex/quick/x86/x86_lir.h
+++ b/compiler/dex/quick/x86/x86_lir.h
@@ -723,7 +723,7 @@
 #define REX_X 0x42
 // Extension of the ModR/M r/m field, SIB base field, or Opcode reg field
 #define REX_B 0x41
-// Extended register set
+// An empty REX prefix used to normalize the byte operations so that they apply to R4 through R15
 #define REX 0x40
 // Mask extracting the least 3 bits of r0..r15
 #define kRegNumMask32 0x07