Cranelift: Remove the LowerCtx trait (#4697)

The trait had only one implementation: the `Lower` struct. It is easier to just
use that directly, and not introduce unnecessary layers of generics and
abstractions.

Once upon a time, there was hope that we would have other implementations of the
`LowerCtx` trait, that did things like lower CLIF to SMTLIB for
verification. However, this is not practical these days given the way that the
trait has evolved over time, and our verification efforts are focused on ISLE
now anyways, and we're actually making some progress on that front (much more
than anyone ever did on a second `LowerCtx` trait implementation!)

cranelift/codegen/src/isa/aarch64/lower_inst.rs

@@ -19,8 +19,8 @@ use core::convert::TryFrom;
 use target_lexicon::Triple;
 
 /// Actually codegen an instruction's results into registers.
-pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
-    ctx: &mut C,
+pub(crate) fn lower_insn_to_regs(
+    ctx: &mut Lower<Inst>,
     insn: IRInst,
     triple: &Triple,
     flags: &Flags,
@@ -39,7 +39,7 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
         return Ok(());
     }
 
-    let implemented_in_isle = |ctx: &mut C| -> ! {
+    let implemented_in_isle = |ctx: &mut Lower<Inst>| -> ! {
         unreachable!(
             "implemented in ISLE: inst = `{}`, type = `{:?}`",
             ctx.dfg().display_inst(insn),
@@ -1621,8 +1621,8 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
     Ok(())
 }
 
-pub(crate) fn lower_branch<C: LowerCtx<I = Inst>>(
-    ctx: &mut C,
+pub(crate) fn lower_branch(
+    ctx: &mut Lower<Inst>,
     branches: &[IRInst],
     targets: &[MachLabel],
 ) -> CodegenResult<()> {