[LangRef] Add noreadafterunwind attribute

[nikic]

Summary

This introduces a noreadafterunwind attribute, which is intended to allow additional optimizations in DSE and MemCpyOpt. The simplest example is:

declare void @may_unwind()
define void @test(i32* noalias noreadafterunwind sret(i32) %out) {
    store i32 0, i32* %out
    call void @may_unwind()
    store i32 1, i32* %out
    ret void
}

Without the noreadafterunwind attribute, we can not eliminate the first store, because the call might unwind, and the caller may observe the state after the first store.

The attribute is currently defined in a way that does not make a read after unwind immediate UB and just makes it return poison instead. I'm open to making it immediate UB though, I don't think there's a lot of practical difference either way.

Frontends will likely want to annotate sret parameters with noreadafterunwind.

See https://lists.llvm.org/pipermail/llvm-dev/2021-December/154164.html for preliminary discussion on this.

[nikic]

Rename to noreadafterunwind to clarify the this only limits reads after the unwind, not that the call itself can't read if it will later unwind.

[nikic]

Abandoning this in favor of D116998.

llvm/docs/LangRef.rst

.. code-block:: llvm
     }
 
     call void @f(i8* @glb, i8* @glb) ; well-defined
 
 ``nofree``
     This indicates that callee does not free the pointer argument. This is not
     a valid attribute for return values.
 
+``noreadafterunwind``
+    This indicates that the underlying object of a pointer argument is not read
+    on the unwind path in the following sense: If a call with a
+    ``noreadafterunwind`` argument unwinds, then the underlying object
+    associated with the argument is overwritten with a :ref:`poison value
+    <poisonvalue>`. As such, reads from this object do not result in immediate
+    undefined behavior, but program behavior cannot depend on the object
+    containing any specific value.  This attribute cannot be applied to return
+    values.
+
 .. _nest:
 
 ``nest``
     This indicates that the pointer parameter can be excised using the
     :ref:`trampoline intrinsics <int_trampoline>`. This is not a valid
     attribute for return values and can only be applied to one parameter.
 
 ``returned``