[Seq] Merge seq lowerings

[mortbopet]

We currently have a lot of seq dialect lowerings. Some of these are probably warranted (i.e. targets different lower-level abstractions than the others), some can probably be merged.

      --lower-seq-fifo                                             -   Lower seq.fifo ops
      --lower-seq-firmem                                           -   Lower seq.firmem ops to instances of hw.module.generated ops
      --lower-seq-firrtl-init-to-sv                                -   Prep the module with macro definitions for firrtl registers.
      --lower-seq-firrtl-to-sv                                     -   Lower sequential firrtl ops to SV.
      --lower-seq-hlmem                                            -   Lowers seq.hlmem operations.
      --lower-seq-to-sv                                            -   Lower sequential ops to SV.

I'd expect a merging of:

• lower-seq-fifo, lower-seq-hlmem, lower-seq-firmem - either lowers Seq ops to other seq ops or hw ops
• lower-seq-to-sv, lower-seq-firrtl-to-sv, lower-seq-firrtl-init-to-sv - lowers (presumably) as low-level-as-possible-seq to sv.

Somewhat also pertains to #5463 (@fabianschuiki).

[fabianschuiki]

This is a great idea! I did a quick attempt at merging some of this a while ago, but that didn't go very far. But it looks like internally these passes are a combination of the dialect conversion framework and global passes over the IR. We should be able to create a pass that does a global sweep over the IR to collect the important stuff, then do a more localized lowering where all the dialect conversion stuff is also called.

[fabianschuiki]

Maybe we should move this into Conversions and create SeqToHW and SeqToSV lowerings as you suggest.