Avoid repeated inlining in FlattenRegUpdate (#1727)

* Avoid repeated inlining in FlattenRegUpdate

When-else structure can lead to the same complex mux structure being
the default on several branches in register update logic. When these are
inlined, it can lead to artifical unreachable branches that show up as
coverage holes in coverage of the emitted Verilog. This commit changes
the inlining logic to prevent inlining any reference expression that
shows up multiple times because this is a common indicator of the
problematic case.

* Add tests for improved register update logic emission

* Improve FlattenRegUpdate comment and add more tests

* [skip formal checks] ICache equivalence check verified locally