[InstCombine] Relax preconditions for ashr+and+icmp fold (PR44754)

[InstCombine] Relax preconditions for ashr+and+icmp fold (PR44754)

Fix for https://bugs.llvm.org/show_bug.cgi?id=44754. We already have
a fold that converts icmp (and (ashr X, C3), C2), C1 into
icmp (and C2'), C1', but it imposed overly strict requirements on the
transform.

Relax this by checking that both C2 and C1 don't shift out bits
(in a signed sense) when forming the new constants.

Alive proofs (https://rise4fun.com/Alive/PTz0):

    Name: ashr_legal
    Pre: ((C2 << C3) >> C3) == C2 && ((C1 << C3) >> C3) == C1
    %a = ashr i16 %x, C3
    %b = and i16 %a, C2
    %c = icmp i16 %b, C1
    =>
    %d = and i16 %x, C2 << C3
    %c = icmp i16 %d, C1 << C3

    Name: ashr_shiftout_eq
    Pre: ((C2 << C3) >> C3) == C2 && ((C1 << C3) >> C3) != C1
    %a = ashr i16 %x, C3
    %b = and i16 %a, C2
    %c = icmp eq i16 %b, C1
    =>
    %c = false

Note that >> corresponds to ashr here. The case of an equality
comparison has some special handling in this transform, because
it will form to a true/false result if the condition on the comparison
constant it violated.

Differential Revision: https://reviews.llvm.org/D74294