Add a work-in-progress backend for x86_64 using the new instruction selection;

Most of the work is credited to Julian Seward.

Co-authored-by: Julian Seward <jseward@acm.org>
Co-authored-by: Chris Fallin <cfallin@mozilla.com>

cranelift/codegen/meta/src/isa/x86/settings.rs

@@ -3,6 +3,12 @@ use crate::cdsl::settings::{PredicateNode, SettingGroup, SettingGroupBuilder};
 pub(crate) fn define(shared: &SettingGroup) -> SettingGroup {
     let mut settings = SettingGroupBuilder::new("x86");
 
+    settings.add_bool(
+        "use_new_backend",
+        "Whether to use the new codegen backend using the new isel",
+        false,
+    );
+
     // CPUID.01H:ECX
     let has_sse3 = settings.add_bool("has_sse3", "SSE3: CPUID.01H:ECX.SSE3[bit 0]", false);
     let has_ssse3 = settings.add_bool("has_ssse3", "SSSE3: CPUID.01H:ECX.SSSE3[bit 9]", false);