[otbn,dv] We don't hit extreme values of immediates in enc_i_cg

These instructions are generated by the StraightLineInsn generator. It may be enough to teach the code that picks immediate operands to favour the boundaries of the range. Grouped into this issue are a couple of other covergroups too:

- [ ] `enc_i_cg`
- [ ] `insn_addi_cg` (we don't add zero to anything)
- [ ] `enc_u_cg`

Start by locating the StraightLineInsn generator and the covergroups named enc_i_cg, insn_addi_cg, and enc_u_cg. Check how immediate operands are selected and how coverage is measured. Done means the generated instructions hit immediate-range boundaries, insn_addi_cg covers adding zero, and all three covergroups are no longer missing coverage.

Written by the indexing model from the issue text.