cmd/compile: catch bad pragma combination earlier

Bad pragmas should never make it to the backend.
I've confirmed manually that the error position is unchanged.

Updates #15756
Updates #19250

Change-Id: If14f7ce868334f809e337edc270a49680b26f48e
Reviewed-on: https://go-review.googlesource.com/38152
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/noder.go

@@ -321,6 +321,9 @@ func (p *noder) funcDecl(fun *syntax.FuncDecl) *Node {
 	if f.Noescape() && len(body) != 0 {
 		yyerror("can only use //go:noescape with external func implementations")
 	}
+	if pragma&Systemstack != 0 && pragma&Nosplit != 0 {
+		yyerror("go:nosplit and go:systemstack cannot be combined")
+	}
 	f.Func.Pragma = pragma
 	lineno = makePos(fun.Pos().Base(), fun.EndLine, 0)
 	f.Func.Endlineno = lineno

src/cmd/compile/internal/gc/pgen.go

@@ -393,9 +393,6 @@ func compile(fn *Node) {
 	}
 	if fn.Func.Pragma&Systemstack != 0 {
 		ptxt.From.Sym.Set(obj.AttrCFunc, true)
-		if fn.Func.Pragma&Nosplit != 0 {
-			yyerror("go:nosplit and go:systemstack cannot be combined")
-		}
 	}
 
 	// Clumsy but important.