[RDA] Only store most recent reaching def from predecessors (NFCI)

When entering a basic block, RDA inserts reaching definitions coming
from predecessor blocks (which will be negative numbers) in a rather
peculiar way. If you have incoming reaching definitions -4, -3, -2, -1,
it will insert those. If you have incoming reaching definitions
-1, -2, -3, -4, it will insert -1, -1, -1, -1, as the max is taken
at each step. That's probably not what was intended...

However, RDA only actually cares about the most recent reaching
definition from a predecessor (to calculate clearance), so this
ends up working fine as far as behavior is concerned. It does
waste memory on unnecessary reaching definitions though.

This patch changes the implementation to first compute the most
recent reaching definition in one loop, and then insert only that
one in a separate loop.

Differential Revision: https://reviews.llvm.org/D77508