[RISCV] Merge RISCVCoalesceVSETVLI back into RISCVInsertVSETVLI (#92869)

We no longer need to separate the passes now that #70549 is landed and
this will unblock #89089.

It's not strictly NFC because it will move coalescing before register
allocation when -riscv-vsetvl-after-rvv-regalloc is disabled. But this
makes it closer to the original behaviour.