X86: Fix type check. Just because an integer type is illegal doesn't mean it's i64.

Fixes PR17495, where an i24 triggered this code. It's intended to
optimize i64 loads on 32 bit x86.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192123 91177308-0d34-0410-b5e6-96231b3b80d8