Simplify applying bitwise operators on sign extended values

[can-leh-emmtrix]

Since the result of comparison and logical operators in C is of type int, complex logical expressions may result in expression trees where sext, extract and logical operators are used together (especially when the programmer uses temporary variables of type int to store boolean values and instcombine has not been executed).

This PR introduces the following simplifications for bit vectors of the same width a and b:

sext(a) & sext(b) -> sext(a & b)
sext(a) | sext(b) -> sext(a | b)
sext(a) ^ sext(b) -> sext(a ^ b)
~sext(a) -> sext(~a)

The pattern extract(sext(x), ...) is already handled by the existing simplifications.

If this breaks other important cases, it might be possible to enforce that a.bits() = b.bits() = 1, although I have personally seen a major impact anywhere else yet.

[nunoplopes]

These rewrite may increase the size of the formula, by adding an additional sext, hence it's not always a win. In general we try to only have local & global optimal rewrites to avoid memory & perf impact.
Do you have any performance numbers?

[nunoplopes]

I've run a bunch of benchmarks over the weekend and this patch doesn't help. Most benchmarks get a bit slower actually.