[cranelift] Simplify SSABuilder with basic block

[Y-Nak]

Feature

Current implementation of SSABuilder has redundant fields and depends on redundant function/struct in order to work with EBB.
We no longer need them and can simplify SSABuilder when assuming all blocks are BasicBlock.

Benefit

Readability and (small) performance improvement.

Implementation

Remove extra fields and functions related to EBB from frontend/src/ssa.rs and frontend/src/frontend.rs.

[Y-Nak]

Draft proposal: https://github.com/Y-Nak/wasmtime/tree/simplify-ssa-builder

[bnjbvr]

Thanks for opening an issue. We identified this simplification opportunity too with @eqrion, so it's definitely something we had in mind. Maybe @eqrion or I could do a proper review for this, if you wanted to submit a PR?

[Y-Nak]

Thanks for responding back! I'll submit a PR.