Disable MSan-hostile loop unswitching.

Loop unswitching may cause MSan false positive when the unswitch
condition is not guaranteed to execute.

This is very similar to ASan and TSan special case in
llvm::isSafeToSpeculativelyExecute (they don't like speculative loads
and stores), but for branch instructions.

This is a workaround for PR28054.

llvm-svn: 272421