[New Optimization Proposal] Propagate conditional result to branch / switch arms

Example:

if (x == 5) {
   return x + 1;
} else {
   return x - 1;
}

if (y == true) { // maxBits(y) <= 1
   return y | x; // maxBits(x) <= 1
} else {
   return y ^ x;
}

if (z < 5) {
   return z >= 5;
}

could be optimize to:

if (x == 5) {
   return 6; // 5 + 1
} else {
   return x - 1;
}

if (y == true) {
   return true; // we know y == true, so true | bool(x)  ->  true
} else {
   return x; // here y always `false` so `false ^ x` -> x
}

if (z < 5) {
   return false;
}

and same for br tables.

WDYT?

Start by examining Binaryen's existing optimization paths for branch and switch arms, including br_table handling. Compare their behavior with the conditional-result examples in the issue and determine the intended transformations. Done means the proposed propagation is implemented consistently for the shown if/else and br_table cases, with tests demonstrating the optimized results.