X86: When emulating unsigned PCMPGTQ with PCMPGTD, fix the sign bit for the smaller type.

Otherwise we'll get a mix of signed and unsigned compares.
Fixes PR15977.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182364 91177308-0d34-0410-b5e6-96231b3b80d8