[RISCV][InsertVSETVLI] Reverse traversal order of block in post pass [nfc]

his unblocks a following change to be more sophisticated during post pass rewriting.

Review wise, I basically just want a second set of eyes. This change should be straight forward, but since it took me an embarrassing number of attempts to get make check to pass. Let's make sure I'm not missing yet another cornercase.

Differential Revision: https://reviews.llvm.org/D139877