cmd/compile: remove incannedimport

This used to be used to give special semantics to the builtin
definitions of package runtime and unsafe, but none of those are
relevant anymore:

- The builtin runtime and unsafe packages do not risk triggering false
  import cycles since they no longer contain `import "runtime"`.

- bimport.go never creates ODCLTYPE, so no need to special case them.

- "incannedimport != 0" is only true when "importpkg != nil" anyway,
  so "incannedimport == 0 && importpkg == nil" is equivalent to just
  "importpkg == nil".

Change-Id: I076f15dd705d4962e7a4c33972e304ef67e7effb
Reviewed-on: https://go-review.googlesource.com/29084
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -289,7 +289,7 @@ func (p *importer) pkg() *Pkg {
 	} else if pkg.Name != name {
 		Yyerror("conflicting package names %s and %s for path %q", pkg.Name, name, path)
 	}
-	if incannedimport == 0 && myimportpath != "" && path == myimportpath {
+	if myimportpath != "" && path == myimportpath {
 		Yyerror("import %q: package depends on %q (import cycle)", importpkg.Path, path)
 		errorexit()
 	}

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

@@ -231,8 +231,6 @@ var funcsyms []*Node
 
 var dclcontext Class // PEXTERN/PAUTO
 
-var incannedimport int
-
 var statuniqgen int // name generator for static temps
 
 var iota_ int32

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

@@ -685,15 +685,12 @@ func loadsys() {
 
 	block = 1
 	iota_ = -1000000
-	incannedimport = 1
 
 	importpkg = Runtimepkg
 	Import(bufio.NewReader(strings.NewReader(runtimeimport)))
 	importpkg = unsafepkg
 	Import(bufio.NewReader(strings.NewReader(unsafeimport)))
-
 	importpkg = nil
-	incannedimport = 0
 }
 
 func importfile(f *Val, indent []byte) {

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

@@ -2096,9 +2096,7 @@ OpSwitch:
 	case ODCLTYPE:
 		ok |= Etop
 		n.Left = typecheck(n.Left, Etype)
-		if incannedimport == 0 {
-			checkwidth(n.Left.Type)
-		}
+		checkwidth(n.Left.Type)
 		break OpSwitch
 	}
 
@@ -2114,7 +2112,7 @@ OpSwitch:
 		}
 	}
 
-	if safemode && incannedimport == 0 && importpkg == nil && compiling_wrappers == 0 && t != nil && t.Etype == TUNSAFEPTR {
+	if safemode && importpkg == nil && compiling_wrappers == 0 && t != nil && t.Etype == TUNSAFEPTR {
 		Yyerror("cannot use unsafe.Pointer")
 	}