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/machinst/inst_common.rs

@@ -1,6 +1,6 @@
 //! A place to park MachInst::Inst fragments which are common across multiple architectures.
 
-use super::{LowerCtx, VCodeInst};
+use super::{Lower, VCodeInst};
 use crate::ir::{self, Inst as IRInst};
 use smallvec::SmallVec;
 
@@ -24,17 +24,14 @@ pub(crate) struct InsnOutput {
     pub(crate) output: usize,
 }
 
-pub(crate) fn insn_inputs<I: VCodeInst, C: LowerCtx<I = I>>(
-    ctx: &C,
-    insn: IRInst,
-) -> SmallVec<[InsnInput; 4]> {
+pub(crate) fn insn_inputs<I: VCodeInst>(ctx: &Lower<I>, insn: IRInst) -> SmallVec<[InsnInput; 4]> {
     (0..ctx.num_inputs(insn))
         .map(|i| InsnInput { insn, input: i })
         .collect()
 }
 
-pub(crate) fn insn_outputs<I: VCodeInst, C: LowerCtx<I = I>>(
-    ctx: &C,
+pub(crate) fn insn_outputs<I: VCodeInst>(
+    ctx: &Lower<I>,
     insn: IRInst,
 ) -> SmallVec<[InsnOutput; 4]> {
     (0..ctx.num_outputs(insn))