[ia32] Make InterpreterEntryTrampoline preserve kRootRegister (ebx)

Also takes care of InterpreterEnterBytecodeAdvance.

Drive-by:
 - Add DCHECK to ensure cmpb is only used with byte registers.
 - Add Push/PopRootRegister to TurboAssembler with DCHECK ensuring
   that there is a NoRootArrayScope


Change-Id: Ibcf1c91b12767bdf6425b18872c41b31124de3ba
Bug: v8:6666
Reviewed-on: https://chromium-review.googlesource.com/1190305Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55512}

src/ia32/assembler-ia32.h

@@ -667,7 +667,10 @@ class V8_EXPORT_PRIVATE Assembler : public AssemblerBase {
   void and_(Operand dst, Register src);
   void and_(Operand dst, const Immediate& x);
 
-  void cmpb(Register reg, Immediate imm8) { cmpb(Operand(reg), imm8); }
+  void cmpb(Register reg, Immediate imm8) {
+    DCHECK(reg.is_byte_register());
+    cmpb(Operand(reg), imm8);
+  }
   void cmpb(Operand op, Immediate imm8);
   void cmpb(Register reg, Operand op);
   void cmpb(Operand op, Register reg);

src/ia32/macro-assembler-ia32.h

@@ -255,6 +255,17 @@ class V8_EXPORT_PRIVATE TurboAssembler : public TurboAssemblerBase {
 
   void LoadAddress(Register destination, ExternalReference source);
 
+  void PushRootRegister() {
+    // Check that a NoRootArrayScope exists.
+    CHECK(!root_array_available());
+    push(kRootRegister);
+  }
+  void PopRootRegister() {
+    // Check that a NoRootArrayScope exists.
+    CHECK(!root_array_available());
+    pop(kRootRegister);
+  }
+
   // Wrapper functions to ensure external reference operands produce
   // isolate-independent code if needed.
   Operand StaticVariable(const ExternalReference& ext);