[dev.simd] cmd/compile: generate function body for bodyless intrinsics

For a compiler intrinsic, if it is used in a non-call context, e.g.
as a function pointer, currently it requires fallback
implementation (e.g. assembly code for atomic operations),
otherwise it will result in a build failure. The fallback
implementation needs to be maintained and tested, albeit rarely
used in practice.

Also, for SIMD, we're currently adding a large number of compiler
intrinsics without providing fallback implementations (we might in
the future). As methods, it is not unlikely that they are used in
a non-call context, e.g. referenced from the type descriptor.

This CL lets the compiler generate the function body for
bodyless intrinsics. The compiler already recognizes a call to
the function as an intrinsic and can directly generate code for it.
So we just fill in the body with a call to the same function.

Change-Id: I2636e3128f28301c9abaf2b48bc962ab56e7d1a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/683096
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/compile/internal/ir/expr.go

@@ -1022,6 +1022,9 @@ func StaticCalleeName(n Node) *Name {
 // IsIntrinsicCall reports whether the compiler back end will treat the call as an intrinsic operation.
 var IsIntrinsicCall = func(*CallExpr) bool { return false }
 
+// IsIntrinsicSym reports whether the compiler back end will treat a call to this symbol as an intrinsic operation.
+var IsIntrinsicSym = func(*types.Sym) bool { return false }
+
 // SameSafeExpr checks whether it is safe to reuse one of l and r
 // instead of computing both. SameSafeExpr assumes that l and r are
 // used in the same statement or expression. In order for it to be
@@ -1140,6 +1143,14 @@ func ParamNames(ft *types.Type) []Node {
 	return args
 }
 
+func RecvParamNames(ft *types.Type) []Node {
+	args := make([]Node, ft.NumRecvs()+ft.NumParams())
+	for i, f := range ft.RecvParams() {
+		args[i] = f.Nname.(*Name)
+	}
+	return args
+}
+
 // MethodSym returns the method symbol representing a method name
 // associated with a specific receiver type.
 //