Cranelift: ensure ISA level needed for SIMD is present when SIMD is enabled. (#3816)

Addresses #3809: when we are asked to create a Cranelift backend with
shared flags that indicate support for SIMD, we should check that the
ISA level needed for our SIMD lowerings is present.

cranelift/src/run.rs

@@ -110,7 +110,7 @@ fn create_target_isa(isa_spec: &IsaSpec) -> Result<Box<dyn TargetIsa>> {
     if let IsaSpec::None(flags) = isa_spec {
         // build an ISA for the current machine
         let builder = host_isa_builder().map_err(|s| anyhow::anyhow!("{}", s))?;
-        Ok(builder.finish(flags.clone()))
+        Ok(builder.finish(flags.clone())?)
     } else {
         anyhow::bail!(
             "A target ISA was specified in the file but should not have been--only \

cranelift/src/utils.rs

@@ -99,7 +99,7 @@ pub fn parse_sets_and_triple(
             .map_err(ParseError::from)?;
 
         Ok(OwnedFlagsOrIsa::Isa(
-            isa_builder.finish(settings::Flags::new(flag_builder)),
+            isa_builder.finish(settings::Flags::new(flag_builder))?,
         ))
     } else {
         if !unknown_settings.is_empty() {