compiler: mark loads of kernel constant attributes as load invariant.

Also, enable LICM, since it can take advantage of this.

artiq/compiler/targets.py

@@ -105,6 +105,7 @@ class Target:
         llpassmgr.add_instruction_combining_pass()
         llpassmgr.add_gvn_pass()
         llpassmgr.add_cfg_simplification_pass()
+        llpassmgr.add_licm_pass()
 
         # Clean up after optimizing.
         llpassmgr.add_dead_arg_elimination_pass()

artiq/compiler/transforms/llvm_ir_generator.py

@@ -796,7 +796,9 @@ class LLVMIRGenerator:
             else:
                 llptr = self.llbuilder.gep(obj, [self.llindex(0), self.llindex(index)],
                                            inbounds=True, name="ptr.{}".format(insn.name))
-                return self.llbuilder.load(llptr, name="val.{}".format(insn.name))
+                llval = self.llbuilder.load(llptr, name="val.{}".format(insn.name))
+                llval.metadata['invariant.load'] = self.empty_metadata
+                return llval
 
     def process_SetAttr(self, insn):
         typ, attr = insn.object().type, insn.attr