[RTG] Support partial sequence substitutions

[maerhart]

No description provided.

[fabianschuiki]

This is a neat way of materializing a symbol as an SSA value. I wonder what other uses this pattern might have 🤔

[maerhart]

One use-case is the context_switch op where you have to pass a sequence with 3 arguments of specific types. If multiple switches have a similar sequence you can create one sequence that has > 3 arguments and partially substitute the first N-3 to specialize for one particular switch instead of copy pasting the whole sequence.