Optimize truth table merge (backport #3993)

[mergify]

I'm collecting numbers, but it should be pretty obvious that not essentially doing a deep copy of every TruthTable both before and after running espresso minimization in the common case where the default value is uniform (i.e. all ones, all zeros, or, most commonly, all dont care) will speed things up.

To implement this, I also added some useful methods to BitPat that you can see the tests for.

Doing split and merge on the TruthTable from TruthTableSpec:

  val table = TruthTable(
    Map(
      // BitPat("b000") -> BitPat("b0"),
      BitPat("b001") -> BitPat("b?"),
      BitPat("b010") -> BitPat("b?"),
      // BitPat("b011") -> BitPat("b0"),
      BitPat("b100") -> BitPat("b1"),
      BitPat("b101") -> BitPat("b1"),
      // BitPat("b110") -> BitPat("b0"),
      BitPat("b111") -> BitPat("b1")
    ),
    BitPat("b0")
  )

Before this change:

Benchmark               Mode  Cnt  Score   Error   Units
ChiselBenchmark.Merge  thrpt   10  0.366 ± 0.003  ops/us
ChiselBenchmark.Split  thrpt   10  0.786 ± 0.006  ops/us

After this change:

Benchmark               Mode  Cnt    Score   Error   Units
ChiselBenchmark.Merge  thrpt   10  453.041 ± 1.422  ops/us
ChiselBenchmark.Split  thrpt   10   83.122 ± 0.184  ops/us

Also from a large real design, I measured it was spending 2 minutes in the little filter function in split:

chisel/src/main/scala/chisel3/util/experimental/decode/TruthTable.scala

Line 145 in d584dd7

def bpFilter(bitPat: BitPat, indexes: Seq[Int]): BitPat =

(this should use a Set instead of a Seq)
With this change, it now spends 0.3ms in that function which just shows how much unnecessary splitting we were doing 🙃.

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
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Type of Improvement

• Performance improvement

Desired Merge Strategy

• Rebase

Release Notes

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This is an automatic backport of pull request #3993 done by [Mergify](https://mergify.com).