AMDGPU: Simplify else if to else in AMDGPUMCCodeEmitter

Fixes llvm#79737

Author: arsenm

llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUMCCodeEmitter.cpp

@@ -402,8 +402,7 @@ void AMDGPUMCCodeEmitter::encodeInstruction(const MCInst &MI,
     else if (Op.isExpr()) {
       if (const auto *C = dyn_cast<MCConstantExpr>(Op.getExpr()))
         Imm = C->getValue();
-
-    } else if (!Op.isExpr()) // Exprs will be replaced with a fixup value.
+    } else // Exprs will be replaced with a fixup value.
       llvm_unreachable("Must be immediate or expr");
 
     if (Desc.operands()[i].OperandType == AMDGPU::OPERAND_REG_IMM_FP64)