[AArch64] [Windows] Address post-commit review comment on r346358.

In this context, usesWindowsCFI() is basically the same thing as
isOSWindows(), but it makes the relevant property of the target
more explicit.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346366 91177308-0d34-0410-b5e6-96231b3b80d8