Improve code generation for smem wmode and [w]mask ports (#834)

[skip formal checks] LEC passes with Formality

* Improve code generation for smem RW-port wmode port

A common case for these port-enables is

    wen = valid & write
    ren = valid & !write

which the RW-port transform currently turns into

    en = (valid & write) | (valid & !write)
    wmode = valid & write

because it proved `wen` and `ren` are mutually exclusive via `write`.

Synthesis tools can trivially optimize `en` to `valid`, so that's not a
problem, but the wmode field can't be optimized if going into a black box.
This PR instead sets `wmode` to whatever node was used to prove
mutual exclusion, which is always a simpler expression.  In this case:

    en = (valid & write) | (valid & !write)
    wmode = write

* In RemoveCHIRRTL, infer mask relative to port definition

Previously, it was inferred relative to the memory definition causing
the mask condition to be redundantly conjoined with the enable signal.

Also enable ReplSeqMems to ignore all ValidIfs (not just on Clocks) to
improve QoR.