cmd/compile: rename Field's Width field to Offset

gorename -from '"cmd/compile/internal/gc".Field.Width' -to Offset

Passes toolstash -cmp.

Change-Id: I310538a1f60bbab470a6375e813e9d5eb52c5bbf
Reviewed-on: https://go-review.googlesource.com/21230
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/src/cmd/compile/internal/gc/gen.go b/src/cmd/compile/internal/gc/gen.go
index b96da80..6a7b64a 100644
--- a/src/cmd/compile/internal/gc/gen.go
+++ b/src/cmd/compile/internal/gc/gen.go
@@ -1235,7 +1235,7 @@
 		}
 
 		for _, field := range t.Fields().Slice() {
-			if !visitComponents(field.Type, startOffset+field.Width, f) {
+			if !visitComponents(field.Type, startOffset+field.Offset, f) {
 				return false
 			}
 		}