[RISCV] Extend dataflow workaround from D119518 to fallthrough blocks

We've got a lurking problem with our data flow implementation where different phases disagree, resulting in possible miscompiles. D119518 introduced a workaround, but failed to consider blocks without terminators (e.g. fallthroughs).

I have a deeper rework of the algorithm in flight over in D125232, but this patch is specifically a minimal fix for an active miscompile. That change can be reworked over this once landed.

Differential Revision: https://reviews.llvm.org/D125408