Fix LDV InstrRefBasedImpl to not crash when encountering unreachable MBBs.

The testcase was delta-reduced from an LTO build with sanitizer
coverage and the MIR tail duplication pass caused a machine basic
block to become unreachable in MIR. This caused the MBB to be invisible
to the reverse post-order traversal used to initialize the MBB <->
RPONumber lookup tables.

rdar://97226240

Differential Revision: https://reviews.llvm.org/D130999