API for asking whether the compiler supports a certain nqp op

This can be used to conditionally compile backend specific code in
modules like NativeCall

src/HLL/Compiler.nqp

@@ -809,6 +809,10 @@ class HLL::Compiler does HLL::Backend::Default {
             nqp::die("The backend { $backend.HOW.name($backend) } doesn't support profiler-snapshot");
         }
     }
+
+    method supports-op($opname) {
+        self.backend.supports-op($opname)
+    }
 }
 
 my $compiler := HLL::Compiler.new();

src/vm/js/HLL/Backend.nqp

@@ -362,6 +362,10 @@ class JavaScriptBackend {
     method is_compunit($cuish) {
         nqp::isinvokable($cuish) || nqp::iscompunit($cuish);
     }
+
+    method supports-op($opname) {
+        0 # NYI, so give the safe answer
+    }
 }
 
 # Role specifying the default backend for this build.

src/vm/jvm/HLL/Backend.nqp

@@ -108,6 +108,10 @@ class HLL::Backend::JVM {
     method compunit_coderefs($cu) {
         nqp::compunitcodes($cu)
     }
+
+    method supports-op($opname) {
+        0 # NYI, so give the safe answer
+    }
 }
 
 # Role specifying the default backend for this build.

src/vm/moar/HLL/Backend.nqp

@@ -1,5 +1,7 @@
 # Backend class for the MoarVM.
 
+use MASTOps;
+
 my sub literal_subst(str $source, str $pattern, $replacement) {
     my $where := 0;
     my $result := $source;
@@ -824,6 +826,10 @@ class HLL::Backend::MoarVM {
     method compunit_coderefs($cu) {
         nqp::compunitcodes($cu)
     }
+
+    method supports-op($opname) {
+        nqp::existskey(%MAST::Ops::codes, $opname) # || MAST::ExtOpRegistry.extop_known($opname)
+    }
 }
 
 # Role specifying the default backend for this build.