Apply knowledge of constraints to SSA constant folding

[TomAFrench]

Currently we do not use the fact that we know a particular value is constrained to be equal to something else in SSA to simplify the circuit.

A glaring example of this is execution_success/references, this is currently represented by the SSA:

After Fill Internal Slice Dummy Data:
acir fn main f0 {
  b0(v0: Field):
    v1383 = add v0, Field 1
    v1384 = eq v1383, Field 3
    constrain v0 == Field 2
    constrain v0 == Field 2
    constrain v0 == Field 2
    constrain v0 == Field 2
    v1385 = eq v1383, Field 10
    v1386 = not v1385
    enable_side_effects v1386
    v1387 = eq v1383, Field 20
    v1388 = not v1387
    v1389 = mul v1386, v1388
    enable_side_effects v1386
    v1390 = cast v1388 as Field
    v1391 = cast v1387 as Field
    v1392 = mul v1390, Field 2
    v1393 = mul v1391, Field 15
    v1394 = add v1392, v1393
    enable_side_effects u1 1
    v1395 = cast v1386 as Field
    v1396 = cast v1385 as Field
    v1397 = mul v1395, v1394
    v1398 = mul v1396, Field 20
    v1399 = add v1397, v1398
    constrain v1399 == Field 2
    v1400 = eq Field 20, v1383
    v1401 = not v1400
    enable_side_effects u1 1
    v1402 = cast v1401 as Field
    v1403 = cast v1400 as Field
    v1404 = mul v1402, Field 15
    v1405 = mul v1403, Field 20
    v1406 = add v1404, v1405
    constrain v1406 == Field 15
    v1407 = not v1384
    enable_side_effects v1407
    v1408 = mul v1407, v1388
    enable_side_effects u1 1
    v1409 = cast v1407 as Field
    v1410 = cast v1384 as Field
    v1411 = mul v1409, v1394
    v1412 = mul v1410, Field 20
    v1413 = add v1411, v1412
    constrain v1413 == Field 20
    enable_side_effects u1 1
    v1414 = mul v1395, Field 2
    v1415 = add v1414, v1398
    constrain v1415 == Field 2
    enable_side_effects v1386
    v1416 = add v1392, v1391
    enable_side_effects u1 1
    v1417 = mul v1395, v1416
    enable_side_effects v1386
    v1418 = mul v1391, v1416
    v1419 = add v1392, v1418
    enable_side_effects v1386
    v1420 = mul v1391, v1419
    v1421 = add v1392, v1420
    enable_side_effects v1386
    v1422 = mul v1391, v1421
    v1423 = add v1392, v1422
    enable_side_effects v1386
    v1424 = mul v1391, v1423
    v1425 = add v1392, v1424
    enable_side_effects u1 1
    v1426 = mul v1395, v1425
    v1427 = add v1426, v1398
    constrain v1427 == Field 2
    enable_side_effects v1386
    v1428 = mul v1386, v1386
    enable_side_effects v1386
    v1429 = mul v1396, v1427
    v1430 = add v1417, v1429
    enable_side_effects v1386
    v1431 = mul v1396, v1430
    v1432 = add v1417, v1431
    enable_side_effects v1386
    v1433 = mul v1396, v1432
    v1434 = add v1417, v1433
    enable_side_effects v1386
    v1435 = mul v1396, v1434
    v1436 = add v1417, v1435
    enable_side_effects v1386
    v1437 = mul v1396, v1436
    v1438 = add v1417, v1437
    enable_side_effects u1 1
    v1439 = mul v1395, v1438
    v1440 = add v1439, v1398
    constrain v1440 == Field 2
    constrain v0 == Field 2 // See this line down here? What if it were at the top?
    return 
}

All of the previous instructions are irrelevant as they all follow from v0 == Field 2. If we knew that at the beginning then we could just remove all of those instructions. We then need to tackle the following tasks:

Tasks

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1. feat: bubble up Instruction::Constrains to be applied as early as possible. #4065 - See example above for motivation. We need to know that v0 is 2 before inserting all of the other instructions. Once that's happened we can...
2. feat: use constraint information to perform constant folding #4060 - Once we've moved the instruction up then we actually need to use this information.