Update fir.insert_on_range syntax to make the range more explicit (NFC)

Also replace ArrayAttr with IndexElementsAttr to model subscript dimensions.
An array of attribute is a sparse inefficient storage, with an API that
requires to unpack/repack integers at every call site.
Instead we can store dense array of integer as IndexElementsAttr.

Reviewed By: clementval, kiranchandramohan

Differential Revision: https://reviews.llvm.org/D112899