Reland "Track field types.".

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index 89f7cd037e28d1942c852311a6577f14c4f539a2..44651cb942b7ac3063d018f04d71cf2cfe7f6806 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -4037,7 +4037,7 @@ void LCodeGen::DoStoreNamedField(LStoreNamedField* instr) {
         Condition cc = masm()->CheckSmi(value);
         DeoptimizeIf(cc, instr->environment());
 
-        // We know that value is a smi now, so we can omit the check below.
+        // We know now that value is not a smi, so we can omit the check below.
         check_needed = OMIT_SMI_CHECK;
       }
     }