[fir] Add fir.zero_bits conversion pattern to LLVM IR dialect

This patch adds the ZeroOpConversion pattern to LLVM IR dialect.
Conversion of aggregate types is not implemented yet and will trigger a
failure to legalize the operation. This is tested in the
convert-to-llvm-invalid.fir test file.

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

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D113014



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