KVM:x86: Clean up ModR/M "reg" initialization in reg op decoding

Refactor decode_register_operand() to get the ModR/M register if and
only if the instruction uses a ModR/M encoding to make it more obvious
how the register operand is retrieved.
Signed-off-by: Liam Ni <zhiguangni01@gmail.com>
Link: https://lore.kernel.org/r/20220908141210.1375828-1-zhiguangni01@zhaoxin.comSigned-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -1137,9 +1137,11 @@ static int em_fnstsw(struct x86_emulate_ctxt *ctxt)
 static void decode_register_operand(struct x86_emulate_ctxt *ctxt,
 				    struct operand *op)
 {
-	unsigned reg = ctxt->modrm_reg;
+	unsigned int reg;
 
-	if (!(ctxt->d & ModRM))
+	if (ctxt->d & ModRM)
+		reg = ctxt->modrm_reg;
+	else
 		reg = (ctxt->b & 7) | ((ctxt->rex_prefix & 1) << 3);
 
 	if (ctxt->d & Sse) {