[FIRRTL] Can not canonicalize away memories without more care

[youngar]

I have some doubts about the validity of the canonicalizer in #7868

This doesn't seem safe to do when we are running with repl-seq-mems, which should mock out the memory with an extmodule. Instead it is lowered to a register and no memory shows up in the metadata.

./bin/firtool --disable-all-randomization --repl-seq-mem --repl-seq-mem-file=mems.txt ./mem.fir

FIRRTL version 4.1.0
circuit Foo:
  public module Foo:
    input clk : Clock
    input r : UInt<1>
    output rdata : UInt<8>
    input wdata : UInt<8>
    input addr : UInt<0>
    input en : UInt<1>
    input mode : UInt<1>


    mem mem:
      data-type => UInt<8>
      depth => 1
      readwriter => rw
      read-latency => 1
      write-latency => 1
      read-under-write => undefined

    connect mem.rw.clk, clk
    connect mem.rw.en, en
    connect mem.rw.addr, addr
    connect mem.rw.wmode, mode
    connect mem.rw.wmask, UInt<8>(-1)
    connect mem.rw.wdata, wdata
    connect rdata, mem.rw.rdata

gives:

// ----- 8< ----- FILE "metadata/seq_mems.json" ----- 8< -----

[]

// ----- 8< ----- FILE "mems.txt" ----- 8< -----

We should also make sure that this is working correctly for cmems, especially that it is giving identical results when we are using MemToRegOfVec, FullAsynchronousResetTransform, PrefixModules, etc.