LiveRangeCalc: Fix reporting of invalid vreg usage in liveness calculation

LiveRangeCalc: Fix reporting of invalid vreg usage in liveness calculation

Machine programs need a definition of each vreg before reaching a use
(the definition may come from an IMPLICIT_DEF instruction). This class
of errors is not detected by the MachineVerifier because of efficiency
concerns. LiveRangeCalc used to report these problems, make it do that
again (followup to r279625).

Also use report_fatal_error() instead of llvm_unreachable() as the error
reporting is only present in asserts build anyway.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@281914 91177308-0d34-0410-b5e6-96231b3b80d8