fix Issue 13417 - segmentation fault when deduce template type
fix Issue 13424 - Initialization of delegate to do-nothing default causes segfault at runtime
Merge pull request #3950 from 9rnsr/fix13417

[REG2.066] Issue 13417 - segmentation fault when deduce template type
Merge pull request #3951 from 9rnsr/fix13424

[REG2.066] Issue 13424 - Initialization of delegate to do-nothing default causes segfault at runtime
Remove un-convertable comment
Do not rely on implicit narrowing integral conversions
Merge pull request #3955 from yebblies/comment4

[DDMD] Remove un-convertable comment
Merge pull request #3954 from yebblies/impconv

[DDMD] Do not rely on implicit narrowing integral conversions
Add TemplateInstance::tnext to construct instantiation dependency graph

To detect speculative instantiations in codegen phase, `tnext` is neccesary for the `test6()` case `runnable/link13350.d`. That's a big difference from the PR #2550.