Change remove-comb-groups to only simplify if-with and while-with

[rachitnigam]

Changes the pass to only transform the if-with and while-with constructs into if and while respectively. Importantly, it leaves invoke-with alone and makes it compile-invoke's responsibility to handle them. This addresses the long open issue of group2invoke -> compile-invoke making the latency of everything worse since group2invoke introduces invoke-with statements while remove-comb-groups turns them into seq { gr; invoke }

Closes #911.

[sampsyo]

Very very awesome! This is some good thinking.