Correct OS conditionals following r204977 and r204978.

Previously, MinGW OS was Triple::MinGW and Cygwin was Triple::Cygwin
and now it is Triple::Win32 with Environment being GNU or Cygwin.
So,

  TheTriple.getOS() == Triple::Win32

is replaced by

  TheTriple.isWindowsMSVCEnvironment()

and

  (TheTriple.getOS() == Triple::MinGW32 || TheTriple.getOS() == Triple::Cygwin)

is replaced by

  TheTriple.isOSCygMing()
Try to fix MSan bootstrap bot: make ARM64Disassembler::getInstruction() always initialize Size argument.