[MachineVerifier] Include offending register in allocatable live-in error msg.

This patch adds a convenience report() method for physical registers and
uses it to print the offending register with the 'MBB has allocatable
live-in' error.

Reviewers: MatzeB, rtereshin, dsanders

Reviewed By: dsanders

Differential Revision: https://reviews.llvm.org/D55946

llvm-svn: 350630