[flang] Lower shift intrinsics

This patch adds lowering for shift intrinsics:
- `ishft`
- `eoshift`
- `ishftc`
- `cshift`

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

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D121808



Co-authored-by: V Donaldson <vdonaldson@nvidia.com>
Co-authored-by: Jean Perier <jperier@nvidia.com>
Co-authored-by: Peter Steinfeld <psteinfeld@nvidia.com>
Co-authored-by: AlexisPerry <aperry@lanl.gov>