WIP: Properly handle const if expression when using proc-scoped channels

[magancarz]

This PR adds handling proper branch from const if when converting DSLX to IR with lower_to_proc_scoped_channels flag enabled. While it works on added ConstConditionalProcScoped test case, it breaks when trying to get a constexpr value from the test expression if const if is inside a parametric proc with the following error:

Error: NOT_FOUND: No constexpr value found for node `CONFIG` (NameRef) @ xls/examples/const_if.x:47:18-47:24

This issue can be reproduced by trying to build the const_if_ir example with changes from this PR.