Optimize truth table merge (backport #3993) (#3995)

* Move BitPatSpec from package chiselTests to package chisel3

This allows it to test package private methods.

(cherry picked from commit 09d6fa71)

* Add package private all* methods to BitPat

(cherry picked from commit e9a2c6d5)

* Optimize TruthTable.split and .merge to do nothing in common case

Most TruthTables have a default of all dont care. We can optimize this
common case by doing actual work in TruthTable.split and
TruthTable.merge.

(cherry picked from commit bb32df47

)

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Co-authored-by: Jack Koenig <koenig@sifive.com>