Bug 885169 - Reverse the default register allocation order so that low registers like eax on x86/x64 are preferred over high registers. r=h4writer

js/src/ion/RegisterSets.h

@@ -396,10 +396,12 @@ class TypedRegisterSet
 #endif
     }
     T getAny() const {
-        JS_ASSERT(!empty());
-        int ireg;
-        JS_FLOOR_LOG2(ireg, bits_);
-        return T::FromCode(ireg);
+        // The choice of first or last here is mostly arbitrary, as they are
+        // about the same speed on popular architectures. We choose first, as
+        // it has the advantage of using the "lower" registers more often. These
+        // registers are sometimes more efficient (e.g. optimized encodings for
+        // EAX on x86).
+        return getFirst();
     }
     T getFirst() const {
         JS_ASSERT(!empty());