[LangRef] Specify that load of alloca outside lifetime is poison

We consider (in bounds) loads from allocas to always be speculatable,
without taking lifetimes into account. This means that such loads
cannot be immediate UB. Specify them as returning poison instead.

Due to stack coloring, such a load may end up loading from a
different alloca, but that's compatible with poison.

Stores are still UB, but that's a much more narrow problem
(I think the only transform violating that part is store scalar
promotion in LICM).

Author: nikic

llvm/docs/LangRef.rst

@@ -3388,6 +3388,10 @@ the object's lifetime. A stack object's lifetime can be explicitly specified
 using :ref:`llvm.lifetime.start <int_lifestart>` and
 :ref:`llvm.lifetime.end <int_lifeend>` intrinsic function calls.
 
+As an exception to the above, loading from a stack object outside its lifetime
+is not undefined behavior and returns a poison value instead. Storing to it is
+still undefined behavior.
+
 .. _pointeraliasing:
 
 Pointer Aliasing Rules