[X86] Remove an if check on the result of a cast. NFC

cast takes a non-null input and produces a non-null output. So this if can never fail.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317731 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed