[opt] Simplify carry extraction pattern (prepending zero bit, adding, then slicing above the original MSb)

[cdleary]

Note the "ZeroExtendedBitsView" in this change seems useful until we come to stronger conclusions on canonical forms (i.e. zero_ext I think should be more canonical than concat as it's a more precise operator, but wouldn't want to make that a prereq).

• Match: bit_slice(add(zext(v), k), start=N, width=1) (carry-out of v + k).
• Rewrite: bit_slice(...) becomes (k[N] ? ult(v, (2^N - (k mod 2^N))) : uge(v, (2^N - (k mod 2^N)))) (and if (k mod 2^N) == 0, becomes literal(k[N])).

before:

$ bazel-bin/xls/tools/opt_main ~/missed_cone.ir 
package cone

top fn cone(leaf_193: bits[8] id=1, leaf_237: bits[1] id=2, leaf_466: bits[1] id=3) -> bits[1] {
  literal.4: bits[1] = literal(value=0, id=4)
  not.6: bits[8] = not(leaf_193, id=6)
  concat.7: bits[9] = concat(literal.4, not.6, id=7)
  literal.8: bits[9] = literal(value=127, id=8)
  add.9: bits[9] = add(concat.7, literal.8, id=9)
  literal.5: bits[8] = literal(value=255, id=5)
  bit_slice.10: bits[1] = bit_slice(add.9, start=8, width=1, id=10)
  eq.20: bits[1] = eq(leaf_193, literal.5, id=20)
  not.13: bits[1] = not(leaf_237, id=13)
  ret and.30: bits[1] = and(bit_slice.10, eq.20, leaf_466, not.13, id=30)
}

which is:

$ ~/proj/xlsynth-crate/target/release/xlsynth-driver ir2gates ~/missed_cone.ir --quiet true
{"live_nodes":28,"deepest_path":7,...}

after:

$ bazel-bin/xls/tools/opt_main ~/missed_cone.ir 
package cone

top fn cone(leaf_193: bits[8] id=1, leaf_237: bits[1] id=2, leaf_466: bits[1] id=3) -> bits[1] {
  ret literal.35: bits[1] = literal(value=0, id=35)
}