cmd/compile: encapsulate Type.Argwid

Passes toolstash -cmp.

Change-Id: I72fb271052e449a83adfa9bd3b923d40781d6341
Reviewed-on: https://go-review.googlesource.com/21243
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/src/cmd/compile/internal/gc/ssa.go b/src/cmd/compile/internal/gc/ssa.go
index d022f2c..3eb462e 100644
--- a/src/cmd/compile/internal/gc/ssa.go
+++ b/src/cmd/compile/internal/gc/ssa.go
@@ -2493,7 +2493,7 @@
 		rcvr = s.newValue1(ssa.OpIData, Types[TUINTPTR], i)
 	}
 	dowidth(fn.Type)
-	stksize := fn.Type.Argwid // includes receiver
+	stksize := fn.Type.ArgWidth() // includes receiver
 
 	// Run all argument assignments. The arg slots have already
 	// been offset by the appropriate amount (+2*widthptr for go/defer,