Add an initial description of a new concept: trap values, and change

Add an initial description of a new concept: trap values, and change
the definition of the nsw and nuw flags to make use of it.

nsw was introduced to help optimizers answer yes to the following:

  // Can we change i from i32 to i64 to eliminate the cast inside the loop?
  for (int i = 0; i < n; ++i) A[i] *= 0.1;

  // Can we assume that this loop will eventually terminate?
  for (int i = 0; i <= n; ++i) A[i] *= 0.1;

In its current form, it isn't truly sufficient for either.

In the first case, if the increment overflows, it'll still have some
valid i32 value; sign-extending it will produce a value which is 33
homogeneous sign bits trailed by 31 independent undef bits. If i is
promoted to i64, it won't have those same values when it reaches that
point. (The compiler could recover here by reasoning about how i is
used by the load, but that's a lot more complicated and isn't always
possible.)

In the second case, there is no value for i which will be greater than
n, so having the increment return undef on overflow doesn't help.

Trap values are a formalization of some existing concepts that we have
about LLVM IR, and give the optimizers a better basis for answering yes
to both questions above.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@102140 91177308-0d34-0410-b5e6-96231b3b80d8