cmd/compile: move testdclstack out of the xdcl_list loop

This is a band-aid, but it fixes the problem
until a deeper fix is in place.

Testing with genpkg -n 50000, I see:

Before:

      154.67 real       184.66 user         3.15 sys

After:

       61.82 real        96.99 user         2.17 sys


Fixes #14781.

Change-Id: I24c7822d60c289bdd6a18a7840b984954c95f7d4
Reviewed-on: https://go-review.googlesource.com/20696Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -2087,10 +2087,6 @@ func (p *parser) xdcl_list() (l []*Node) {
 			continue
 		}
 
-		if nsyntaxerrors == 0 {
-			testdclstack()
-		}
-
 		// Reset p.pragma BEFORE advancing to the next token (consuming ';')
 		// since comments before may set pragmas for the next function decl.
 		p.pragma = 0
@@ -2100,6 +2096,10 @@ func (p *parser) xdcl_list() (l []*Node) {
 			p.advance(LVAR, LCONST, LTYPE, LFUNC)
 		}
 	}
+
+	if nsyntaxerrors == 0 {
+		testdclstack()
+	}
 	return
 }