Use a fixed input register where we require a byte register.

src/ia32/lithium-ia32.cc

Index: src/ia32/lithium-ia32.cc
===================================================================
--- src/ia32/lithium-ia32.cc	(revision 7408)
+++ src/ia32/lithium-ia32.cc	(working copy)
@@ -1905,7 +1905,6 @@
   ASSERT(instr->key()->representation().IsInteger32());
 
   LOperand* external_pointer = UseRegister(instr->external_pointer());
-  LOperand* val = UseRegister(instr->value());
   LOperand* key = UseRegister(instr->key());
   LOperand* temp = NULL;
 
@@ -1916,6 +1915,15 @@
     temp = FixedTemp(eax);
   }
 
+  LOperand* val = NULL;
+  if (array_type == kExternalByteArray ||
+      array_type == kExternalUnsignedByteArray) {
+    // We need a byte register in this case for the value.
+    val = UseFixed(instr->value(), eax);
+  } else {
+    val = UseRegister(instr->value());
+  }
+
   return new LStoreKeyedSpecializedArrayElement(external_pointer,
                                                 key,
                                                 val,