[MachineVerifier] Handle the PHI node for verifyLiveVariables()

Summary:
When doing MachineVerifier for LiveVariables, the MachineVerifier pass
will calculate the LiveVariables, and compares the result with the
result livevars pass gave. If they are different, verifyLiveVariables()
will give error.

But when we calculate the LiveVariables in MachineVerifier, we don't
consider the PHI node, while livevars considers.

This patch is to fix above bug.

Reviewed By: bjope

Differential Revision: https://reviews.llvm.org/D80274