[LivePhysRegs] Preserve pristine regs in blocks with no successors.

One common source of blocks with no successors is calls to noreturn
functions; we want to preserve pristine registers in case they throw an
exception.

The whole pristine register thing is messy (we should really prefer to
explicitly model registers), but this fills a hole in the model for now.

Fixes https://bugs.llvm.org/show_bug.cgi?id=36073.

Differential Revision: https://reviews.llvm.org/D42509

llvm-svn: 323559