[SOL] Do not add a return after a JA

[LucasSte]

We can simplify the patch of function with the return instruction, if we know that last MachineBasicBlock is also the last block in the ELF layout.

Since PreEmitPeephole happens after block placement, that assumption holds.

Also, I avoided adding return when the last instruction is a ja.

[Lichtso]

Isn't this very niche? It can only ever optimize a single instruction, right?

[LucasSte]

Isn't this very niche? It can only ever optimize a single instruction, right?

This PR simplifies a couple of things:

1. I was adding a return to every terminator block that didn't end in return. We do not need to do that for every block, but only the last one in the ELF layout. Hence, I removed the iteration.
2. The return is not necessary if the last instruction is a ja.