[ARM] Fix a warning

This patch fixes:

  llvm/lib/Target/ARM/ARMAsmPrinter.cpp:1643:9: error: unused variable
  'T2SOImmVal' [-Werror,-Wunused-variable]

Author: kazutakahirata

llvm/lib/Target/ARM/ARMAsmPrinter.cpp

@@ -1640,8 +1640,7 @@ void ARMAsmPrinter::EmitKCFI_CHECK_Thumb2(Register AddrReg, int64_t Type,
     bool isLast = (i == 3);
 
     // Verify the immediate can be encoded as Thumb2 modified immediate.
-    int T2SOImmVal = ARM_AM::getT2SOImmVal(imm);
-    assert(T2SOImmVal != -1 &&
+    assert(ARM_AM::getT2SOImmVal(imm) != -1 &&
            "Cannot encode immediate as Thumb2 modified immediate");
 
     // eor[s] scratch, scratch, #imm (last one sets flags with CPSR)