better const folding for int and bool

[Firestar99]

see #300

const folding used to only support:

• basic int: add, mul
• casting from Optimize constant casts to avoid unnecessary capabilities #302

This PR adds const folding for:

• basic int: sub, div, rem, shr, lshl, ashl, neg, not (+ unchecked variants)
• basic bool: and, or, xor, not
• comparison for int and bool (icmp)
• select with const condition

Should probably add some diff tests

[Firestar99]

@eddyb Do you have any explanation why 43d5107 is required to make libm compile? This PR also adds explicit type checking for each operation, and for some reason libm calls add that meant for integers with two floats instead of calling fadd.