[Opt] Add TI_DUMP_CFG to dump CFG graph during optimization passes

[hughperkins]

Issue: #

Brief Summary

Honor TI_DUMP_CFG=1 to dump CFG graph during optimization passes

• dumps the statements as well as the graph, to facilitae reading
• dumps to file to facilitate browsing through various cfg graphs, and also easily usable by Cursor
• handles adding phase name to filename, so multiple passes don't overwrite each other

Using TI_DUMP_CFG instad of TI_DUMP_IR or TI_DUMP_SIMPLIFY, because these are fairly specialist files, and no point in writing them out systematically with TI_DUMP_IR I feel.

Example outputs:

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copilot:summary

Walkthrough

copilot:walkthrough