machinst: allow passing constant information to the instruction emitter;

A new associated type Info is added to MachInstEmit, which is the
immutable counterpart to State. It can't easily be constructed from an
ABICallee, since it would require adding an associated type to the
latter, and making so leaks the associated type in a lot of places in
the code base and makes the code harder to read. Instead, the EmitInfo
state can simply be passed to the `Vcode::emit` function directly.

cranelift/codegen/src/isa/aarch64/inst/emit_tests.rs

@@ -9,7 +9,6 @@ use alloc::vec::Vec;
 
 #[test]
 fn test_aarch64_binemit() {
-    let flags = settings::Flags::new(settings::builder());
     let mut insns = Vec::<(Inst, &str, &str)>::new();
 
     // N.B.: the architecture is little-endian, so when transcribing the 32-bit
@@ -4668,7 +4667,9 @@ fn test_aarch64_binemit() {
 
     insns.push((Inst::Fence {}, "BF3B03D5", "dmb ish"));
 
-    let rru = create_reg_universe(&settings::Flags::new(settings::builder()));
+    let flags = settings::Flags::new(settings::builder());
+    let rru = create_reg_universe(&flags);
+    let emit_info = EmitInfo::new(flags);
     for (insn, expected_encoding, expected_printing) in insns {
         println!(
             "AArch64: {:?}, {}, {}",
@@ -4681,7 +4682,7 @@ fn test_aarch64_binemit() {
 
         let mut sink = test_utils::TestCodeSink::new();
         let mut buffer = MachBuffer::new();
-        insn.emit(&mut buffer, &flags, &mut Default::default());
+        insn.emit(&mut buffer, &emit_info, &mut Default::default());
         let buffer = buffer.finish();
         buffer.emit(&mut sink);
         let actual_encoding = &sink.stringify();