InstCombine: Properly optimize or'ing bittests together

CFE, with -03, would turn:
bool f(unsigned x) {
  bool a = x & 1;
  bool b = x & 2;
  return a | b;
}

into:
  %1 = lshr i32 %x, 1
  %2 = or i32 %1, %x
  %3 = and i32 %2, 1
  %4 = icmp ne i32 %3, 0

This sort of thing exposes a nasty pathology in GCC, ICC and LLVM.

Instead, we would rather want:
  %1 = and i32 %x, 3
  %2 = icmp ne i32 %1, 0

Things get a bit more interesting in the following case:
  %1 = lshr i32 %x, %y
  %2 = or i32 %1, %x
  %3 = and i32 %2, 1
  %4 = icmp ne i32 %3, 0

Replacing it with the following sequence is better:
  %1 = shl nuw i32 1, %y
  %2 = or i32 %1, 1
  %3 = and i32 %2, %x
  %4 = icmp ne i32 %3, 0

This sequence is preferable because %1 doesn't involve %x and could
potentially be hoisted out of loops if it is invariant; only perform
this transform in the non-constant case if we know we won't increase
register pressure.