[LangRef] Mention that freeze does not consider aggregate's paddings

[aqjune]

Summary

Make explicit that freeze does not touch paddings of an aggregate.
(Relevant comment: https://reviews.llvm.org/D83752#2152550)

This implies that v = freeze(load p); store v, q may still leave undef bits
or poison in memory if v is an aggregate, but it still happens for
non-byte integers such as i1.

[nikic]

LG

llvm/docs/LangRef.rst

 read from arbitrary registers that happen to be around when needed, so
 the value is not necessarily consistent over time. In fact, ``%A`` and
 ``%C`` need to have the same semantics or the core LLVM "replace all
 uses with" concept would not hold.
 
 To ensure all uses of a given register observe the same value (even if
 '``undef``'), the :ref:`freeze instruction <i_freeze>` can be used.
 A value is frozen if its uses see the same value.
+An aggregate value or vector is frozen if its elements are frozen.
+The padding of an aggregate isn't considered, since it isn't visible
+without storing it into memory and loading it with a different type.
 
 .. code-block:: llvm
 
       %A = sdiv undef, %X
       %B = sdiv %X, undef
     Safe:
       %A = 0
     b: unreachable

 Otherwise, this instruction is a no-op and returns the input argument.
 All uses of a value returned by the same '``freeze``' instruction are
 guaranteed to always observe the same value, while different '``freeze``'
 instructions may yield different values.
 
 While ``undef`` and ``poison`` pointers can be frozen, the result is a
 non-dereferenceable pointer. See the
 :ref:`Pointer Aliasing Rules <pointeraliasing>` section for more information.
+If an aggregate value or vector is frozen, the operand is frozen element-wise.
+The padding of an aggregate isn't considered, since it isn't visible
+without storing it into memory and loading it with a different type.
 
 
 Example:
 """"""""
 
 .. code-block:: text
 
       %w = i32 undef