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/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() {