[fir] Add fir.cmpc conversion

This patch adds the codegen for fir.cmpc. The real and imaginary parts
are extracted and compared separately. For the .EQ. predicate the
results are AND'd, for the .NE. predicate the results are OR'd, and for
other predicates we keep only the result on the real parts.

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

Differential Revision: https://reviews.llvm.org/D113976



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