[fir] Update fir.alloca op

Update the fir.alloca operation.

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

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D110415



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