Enforce stricter liveness rules for PHIs.

Verify that all paths from the entry block to a virtual register read
pass through a def. Enable this check even when MRI->isSSA() is false.

Verify that the live range of a virtual register is live out of all
predecessor blocks, even for PHI-values.

This requires that PHIElimination sometimes inserts IMPLICIT_DEF
instruction in predecessor blocks.

llvm-svn: 159150