[X86] Don't model UD2/UD2B as a terminator

A UD2 might make its way into the program via a call to @llvm.trap.
Obviously, calls are not terminators.  However, we modeled the X86
instruction, UD2, as a terminator.  Later on, this confuses the epilogue
insertion machinery which results in the epilogue getting inserted
before the UD2.  For some platforms, like x64, the result is a
violation of the ABI.

Instead, model UD2/UD2B as a side effecting instruction which may
observe memory.

llvm-svn: 278144