Rework LLVM Dialect LoopOptions attribute

Instead of storing an array of LoopOpt attributes, which were just
wrapping std::pair<enum, int> anyway, we can have an attribute storing
a sorted ArrayRef<std::pair<enum, int>> as a single unit. This improves
here the textual format and the general API. Note that we're limiting
the options to fit into an int64_t by design, but this isn't a new
constraint.

Building the LoopOptions attribute is likely worth a specific builder
for efficient reason, that'll be the subject of a future patch.

Differential Revision: https://reviews.llvm.org/D98105