[clang][NFC] Assert not llvm_unreachable (#70149)

An assert is better here.

Author: urnathan

clang/lib/CodeGen/CGExprScalar.cpp

@@ -2084,11 +2084,10 @@ Value *ScalarExprEmitter::VisitCastExpr(CastExpr *CE) {
     Value *Src = Visit(const_cast<Expr*>(E));
     llvm::Type *SrcTy = Src->getType();
     llvm::Type *DstTy = ConvertType(DestTy);
-    if (SrcTy->isPtrOrPtrVectorTy() && DstTy->isPtrOrPtrVectorTy() &&
-        SrcTy->getPointerAddressSpace() != DstTy->getPointerAddressSpace()) {
-      llvm_unreachable("wrong cast for pointers in different address spaces"
-                       "(must be an address space cast)!");
-    }
+    assert(
+        (!SrcTy->isPtrOrPtrVectorTy() || !DstTy->isPtrOrPtrVectorTy() ||
+         SrcTy->getPointerAddressSpace() == DstTy->getPointerAddressSpace()) &&
+        "Address-space cast must be used to convert address spaces");
 
     if (CGF.SanOpts.has(SanitizerKind::CFIUnrelatedCast)) {
       if (auto *PT = DestTy->getAs<PointerType>()) {