X86: simplify data layout calculation

X86Subtarget::isTargetCygMing || X86Subtarget::isTargetKnownWindowsMSVC is
equivalent to all Windows environments.  Simplify the check to isOSWindows.
NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210431 91177308-0d34-0410-b5e6-96231b3b80d8