[IR] Specify alloca with poison element count (llvm#183072)

An alloca with a poison element count is undefined behavior.

This matches existing behavior of optimizations. This also matches the
behavior of llubui for `poison`, but llubi currently does not report
immediate UB for `undef` element counts. A future patch will fix that.

Author: boomanaiden154

llvm/docs/LangRef.rst

@@ -11613,6 +11613,9 @@ See :ref:`llvm.lifetime.start <int_lifestart>` and
 :ref:`llvm.lifetime.end <int_lifeend>` for the precise semantics of
 lifetime-manipulating intrinsics.
 
+If the element count is ``undef`` or ``poison``, this instruction has undefined
+behavior.
+
 Example:
 """"""""