[fir][NFC] Update fir.iterate_while op

Add getFinalValueAttrName() and remove specified number of
inlined elements for SmallVector. This patch is mainly motivated
to help the upstreaming effort.

This patch is part of the upstreaming effort from fir-dev branch.

Co-authored-by: Jean Perier <jperier@nvidia.com>
Co-authored-by: Valentin Clement <clementval@gmail.com>

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D110710