[dev.regabi] cmd/compile: cleanup label handling

- The use of a label's Name.Defn to point at the named for/select/switch
  means that any rewrite of the for/select/switch must overwrite the original
  or else the pointer will dangle. Remove that pointer by adding the label
  name directly to the for/select/switch representation instead.

- The only uses of a label's Sym.Label were ephemeral values during
  markbreak and escape analysis. Use a map for each. Although in general
  we are not going to replace all computed fields with maps (too slow),
  the one in markbreak is only for labeled for/select/switch, and the one
  in escape is for all labels, but even so, labels are fairly rare.

In theory this cleanup should make it easy to allow labeled for/select/switch
in inlined bodies, but this CL does not attempt that. It's only concerned
with cleanup to enable a new Node representation.

Passes buildall w/ toolstash -cmp.

Change-Id: I7e36ee98d2ea40dbae94e6722d585f007b7afcfa
Reviewed-on: https://go-review.googlesource.com/c/go/+/274086
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/cmd/compile/internal/types/sizeof_test.go b/src/cmd/compile/internal/types/sizeof_test.go
index 2821d9a3c7..88a2fbba2f 100644
--- a/src/cmd/compile/internal/types/sizeof_test.go
+++ b/src/cmd/compile/internal/types/sizeof_test.go
@@ -20,7 +20,7 @@ func TestSizeof(t *testing.T) {
 		_32bit uintptr     // size on 32bit platforms
 		_64bit uintptr     // size on 64bit platforms
 	}{
-		{Sym{}, 60, 104},
+		{Sym{}, 52, 88},
 		{Type{}, 56, 96},
 		{Map{}, 20, 40},
 		{Forward{}, 20, 32},