JIT: Examples where long decomposition could be improved

[AndyAyersMS]

On x86 (and presumably, arm32), operations on longs must be broken down into sets of operations on ints. This process can sometimes introduce unnecessary register pressure and lead to poor CQ.

Some examples:

• For indirs, it would be better to reuse addresses: https://github.com/dotnet/coreclr/issues/18339#issuecomment-468355602
• For copies, it would be better not to create overlapping lifetimes: JIT: detect address of field as an invariant inlining arg #845 (comment)

category:cq
theme:ir
skill-level:intermediate
cost:medium

[AndyAyersMS]

I believe this is still the case.

@SingleAccretion this might be something for a rainy day.