[WinEHPrepare] Replace unreasonable funclet terminators with unreachable

It is possible to be in a situation where more than one funclet token is
a valid SSA value.  If we see a terminator which exits a funclet which
doesn't use the funclet's token, replace it with unreachable.

Differential Revision: http://reviews.llvm.org/D12074

llvm-svn: 245238