[LangRef] Specify that load of alloca outside lifetime is poison (#157852)

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).

Fixes #141892 (and probably a
bunch of others...)

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