Cosmetic (ARM/MIPS): Use FieldMemOperand where appropriate.

src/mips/full-codegen-mips.cc

Index: src/mips/full-codegen-mips.cc
===================================================================
--- src/mips/full-codegen-mips.cc	(revision 11205)
+++ src/mips/full-codegen-mips.cc	(working copy)
@@ -2977,7 +2977,7 @@
     __ Move(f14, zero_reg, a1);
     // Subtract and store the result in the heap number.
     __ sub_d(f0, f12, f14);
-    __ sdc1(f0, MemOperand(s0, HeapNumber::kValueOffset - kHeapObjectTag));
+    __ sdc1(f0, FieldMemOperand(s0, HeapNumber::kValueOffset));
     __ mov(v0, s0);
   } else {
     __ PrepareCallCFunction(2, a0);