arch-x86: Don't decode SIB bytes in 32 bit mode.

There was a comment there saying we shouldn't, but then we still did
anyway.

Change-Id: I4a53cf504d38e00fca5d687818149b91354e640d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/55593
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>

src/arch/x86/decoder.cc

@@ -544,8 +544,7 @@ Decoder::doModRMState(uint8_t nextByte)
 
     // If there's an SIB, get that next.
     // There is no SIB in 16 bit mode.
-    if (modRM.rm == 4 && modRM.mod != 3) {
-            // && in 32/64 bit mode)
+    if (modRM.rm == 4 && modRM.mod != 3 && emi.addrSize != 2) {
         nextState = SIBState;
     } else if (displacementSize) {
         nextState = DisplacementState;