LoopRotate: Fix avoiding copy_value hoisted to the pre-header

[meg-gupta]

copy_value is marked as having no side effects.
Teach LoopRotate to avoid moving it to the pre header and instead duplicate it while rotating the loop.

[meg-gupta]

@swift-ci test

[atrick]

How does loop rotate handle other instructions which "may retain"?
Isn't copy_value considered to have a "retain" side effect? If not, that seems like a bigger bug.

[meg-gupta]

LoopRotate looks for mayHaveSideEffects - https://github.com/apple/swift/blob/5b9aef91f8e96dcc82ac3828c18fa26473d17fe6/lib/SILOptimizer/LoopTransforms/LoopRotate.cpp#L116

copy_value is marked as having no side effects

[atrick]

Meghana pointed out that in copy_value is not actually supposed to have side effects. I think this change is fine and we can discuss how to move forward in parallel

[atrick]

@eeckstein Here's the issue:

In OSSA copy_value does not have any side-effects for the purpose of deleting the instruction or moving code around the instructions. As long as the OSSA constraints still hold-up, those transformations are legal. So, in that sense "may-retain" is false. But for the purpose of hoisting/sinking the copy_value itself, it does perform a retain. So in that sense, "may-retain" is true. Any idea how we want to model this kind of side-effect going forward?