[mlir] NFC: Rename LoopOps dialect to SCF (Structured Control Flow)

This dialect contains various structured control flow operaitons, not only
loops, reflect this in the name. Drop the Ops suffix for consistency with other
dialects.

Note that this only moves the files and changes the C++ namespace from 'loop'
to 'scf'. The visible IR prefix remains the same and will be updated
separately. The conversions will also be updated separately.

Differential Revision: https://reviews.llvm.org/D79578