machinst x64: Only use the feature flag to enable the x64 new backend;

Before this patch, running the x64 new backend would require both
compiling with --features experimental_x64 and running with
`use_new_backend`.

This patches changes this behavior so that the runtime flag is not
needed anymore: using the feature flag will enforce usage of the new
backend everywhere, making using and testing it much simpler:

    cargo run --features experimental_x64 ;; other CLI options/flags

This also gives a hint at what the meta language generation would look
like after switching to the new backend.

Compiling only with the x64 codegen flag gives a nice compile time speedup.

cranelift/codegen/meta/src/isa/mod.rs

@@ -6,10 +6,10 @@ use std::fmt;
 mod arm32;
 mod arm64;
 mod riscv;
-mod x86;
+pub(crate) mod x86;
 
 /// Represents known ISA target.
-#[derive(Copy, Clone)]
+#[derive(PartialEq, Copy, Clone)]
 pub enum Isa {
     Riscv,
     X86,

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

@@ -14,7 +14,7 @@ mod legalize;
 mod opcodes;
 mod recipes;
 mod registers;
-mod settings;
+pub(crate) mod settings;
 
 pub(crate) fn define(shared_defs: &mut SharedDefinitions) -> TargetIsa {
     let settings = settings::define(&shared_defs.settings);

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

@@ -3,12 +3,6 @@ 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);