opt_expr: Add more $alu optimizations.

opt_expr: Add more $alu optimizations.

Detect the places in the $alu where the carry bit is constant (due to
const A[i] == B[i] ^ BI) and split it into smaller $alu at these points.

Also, make the existing const-carry detection for low bits more generic
(now handles cases where both BI and CI are constant, but not equal to
one another).

Fixes #1912.