[DAGCombine][X86][AArch64] 'A - (A & (B - 1))' -> 'A & (0 - B)' fold (PR44448)
While we do manage to fold integer-typed IR in middle-end,
we can't do that for the main motivational case of pointers.
There is @llvm.ptrmask() intrinsic which may or may not be helpful,
but i'm not sure it is fully considered canonical yet,
not everything is fully aware of it likely.
Name: ptr - (ptr & (alignment-1)) -> ptr & (0 - alignment)
%mask = add i64 %alignment, -1 %bias = and i64 %ptr, %mask %r = sub i64 %ptr, %bias
%highbitmask = sub i64 0, %alignment %r = and i64 %ptr, %highbitmask