[llvm] [Docs] Update frontend perfomance tips about loads/stores (PR #83833)

[Ralf Jung via llvm-commits]

================
@@ -64,16 +64,36 @@ SSA is the canonical form expected by much of the optimizer; if allocas can
 not be eliminated by Mem2Reg or SROA, the optimizer is likely to be less
 effective than it could be.
 
-Avoid loads and stores of large aggregate type
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Avoid creating values of aggregate type
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Avoid creating values of :ref:`aggregate types <t_aggregate>` (i.e. structs and
+arrays). In particular, avoid loading and storing them, or manipulating them
+with insertvalue and extractvalue instructions. Instead, only load and store
+individual fields of the aggregate.
+
+There are some exceptions to this rule:
+
+* It is fine to use values of aggregate type in global variable initializers.
+* It is fine to return structs, if this is done to represent the return of
+  multiple values in registers.
+* It is fine to work with structs returned by LLVM intrinsics, such as the
+  ``with.overflow`` family of intrinsics.
+* It is fine to use aggregate *types* without creating values. For example,
+  they are commonly used in ``getelementptr`` instructions or attributes like
+  ``sret``.
+
+Avoid loads and stores of non-byte-sized types
----------------
RalfJung wrote:

Is "non-byte-sized" a standard term? I first thought it means "types of a size that is not exactly one byte". But I'm also not super familiar with standard LLVM terminology.

The text inside this section makes it clear enough.

https://github.com/llvm/llvm-project/pull/83833