Simplify LowerBackend interface (#5432)

* Refactor lower_branch to have Unit result

Branches cannot have any output, so it is more straightforward
to have the ISLE term return Unit instead of InstOutput.

Also provide a new `emit_side_effect` term to simplify
implementation of `lower_branch` rules with Unit result.

* Simplify LowerBackend interface

Move all remaining asserts from the LowerBackend::lower and
::lower_branch_group into the common call site.

Change return value of ::lower to Option<InstOutput>, and
return value of ::lower_branch_group to Option<()> to match
ISLE term signature.

Only pass the first branch into ::lower_branch_group and
rename it to ::lower_branch.

As a result of all those changes, LowerBackend routines
now consists solely to calls to the corresponding ISLE
routines.

cranelift/codegen/src/isa/x64/lower/isle.rs

@@ -68,7 +68,7 @@ pub(crate) fn lower_branch(
     backend: &X64Backend,
     branch: Inst,
     targets: &[MachLabel],
-) -> Option<InstOutput> {
+) -> Option<()> {
     // TODO: reuse the ISLE context across lowerings so we can reuse its
     // internal heap allocations.
     let mut isle_ctx = IsleContext { lower_ctx, backend };