[LiveVariables] Don't set undef reg PHI used as live for FromMBB

Summary:
In the patch D73152, it adds a new function LiveVariables::addNewBlock.
This new function will add the reg which PHI used to the MBB which reg
is from.
But the new function may cause LiveVariable Verification failed when the
Src reg in PHI is undef.

Reviewed By: bjope

Differential Revision: https://reviews.llvm.org/D80077