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/machinst/isle.rs

@@ -140,7 +140,7 @@ macro_rules! isle_lower_prelude_methods {
                         _ => None,
                     };
                     if let Some(insn) = insn {
-                        if let Ok(regs) = self.backend.lower(self.lower_ctx, insn) {
+                        if let Some(regs) = self.backend.lower(self.lower_ctx, insn) {
                             assert!(regs.len() == 1);
                             return regs[0];
                         }