[PS4] Guarantee an instruction after a 'noreturn' call.

We need the "return address" of a noreturn call to be within the
bounds of the calling function; TrapUnreachable turns 'unreachable'
into a 'ud2' instruction, which has that desired effect.

Differential Revision: http://reviews.llvm.org/D18414

llvm-svn: 264224