[fir] Add IfBuilder and utility functions

In order to reduct the size of D111337. The IfBuilder and the two
utility functions genIsNotNull and genIsNull have been extracted in
a separate patch with dedicated unittests.

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

Reviewed By: Leporacanthicus

Differential Revision: https://reviews.llvm.org/D111796



Co-authored-by: Jean Perier <jperier@nvidia.com>
Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>