[X86] Place parentheses around "isMask_32(STReturns) && N <= 2".

This corrects r214672, which was committed to silence a gcc warning.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214732 91177308-0d34-0410-b5e6-96231b3b80d8