[Docs] Update frontend perfomance tips about loads/stores

[nikic]

This contains two updates:

• Generalize the "no loads stores of aggregates" to "no values of aggregate type" in general, and be clearer about the exceptions where it is okay to use them.
• Mention that you should not load/store non-byte-size types.

[RalfJung]

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.

[nikic]

I think it's a pretty common term. In the backend we use isByteSized, in the middle-end typeSizeEqualsStoreSize to query these.

[preames]

LGTM