[llvm] [MC][x86] Fix missing check in MC binary expression (PR #75693)

[Chenyang Gao via llvm-commits]

https://github.com/cygao90 created https://github.com/llvm/llvm-project/pull/75693

This fixes #73109.
In instruction `addl %eax %rax`, because there is a missing comma in the middle of two registers, the asm parser will treat it as a binary expression.
```
%rax  %  rax --> register mod identifier
```
However, In `MCExpr::evaluateAsRelocatableImpl`, it only checks the left side of the expression. This patch ensures the right side will also be checked.

>From cd8e4f760c50430a4b2826a71039490247ae0d21 Mon Sep 17 00:00:00 2001
From: Chenyang Gao <cygao09 at gmail.com>
Date: Sat, 16 Dec 2023 15:35:27 +0800
Subject: [PATCH] [MC] Fix missing check in binary expression

---
 llvm/lib/MC/MCExpr.cpp | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/llvm/lib/MC/MCExpr.cpp b/llvm/lib/MC/MCExpr.cpp
index 73e6569f96e463..4bed3d2d7a1c94 100644
--- a/llvm/lib/MC/MCExpr.cpp
+++ b/llvm/lib/MC/MCExpr.cpp
@@ -942,16 +942,17 @@ bool MCExpr::evaluateAsRelocatableImpl(MCValue &Res, const MCAssembler *Asm,
                                                   Addrs, InSet)) {
       // Check if both are Target Expressions, see if we can compare them.
       if (const MCTargetExpr *L = dyn_cast<MCTargetExpr>(ABE->getLHS())) {
-        const MCTargetExpr *R = cast<MCTargetExpr>(ABE->getRHS());
-        switch (ABE->getOpcode()) {
-        case MCBinaryExpr::EQ:
-          Res = MCValue::get(L->isEqualTo(R) ? -1 : 0);
-          return true;
-        case MCBinaryExpr::NE:
-          Res = MCValue::get(L->isEqualTo(R) ? 0 : -1);
-          return true;
-        default:
-          break;
+        if (const MCTargetExpr *R = dyn_cast<MCTargetExpr>(ABE->getRHS())) {
+          switch (ABE->getOpcode()) {
+          case MCBinaryExpr::EQ:
+            Res = MCValue::get(L->isEqualTo(R) ? -1 : 0);
+            return true;
+          case MCBinaryExpr::NE:
+            Res = MCValue::get(L->isEqualTo(R) ? 0 : -1);
+            return true;
+          default:
+            break;
+          }
         }
       }
       return false;