[opt] Reduction under mask -- sign_ext elimination

[cdleary]

From the block comment:

  // Reduction of a value masked by sign_ext(bits[1]) (eliminate mask plumbing).
  //
  // Note on abbreviation in the pattern below:
  //
  // - `w` is the bit-width of the masked datapath value `x` (and thus the
  //   width of the `and(x, mask)`).
  // - `b` is the 1-bit predicate (bits[1]) being sign-extended to a `w`-bit
  //   mask via `sign_ext(b, w)` (all-zeros when b=0, all-ones when b=1).
  //
  // For w > 1, sign_ext(b:bits[1], w) is either 0_w or all_ones_w, so:
  //
  //   or_reduce(and(x, sign_ext(b,w)))  == and(b, or_reduce(x))
  //   and_reduce(and(x, sign_ext(b,w))) == and(b, and_reduce(x))
  //
  // And similarly for the inverted mask not(sign_ext(b,w)).
  //
  // This is always semantics-preserving and strictly reduces mask plumbing for
  // w > 1.