cmd/compile: merge TPTR32 and TPTR64 as TPTR

Change-Id: I0490098a7235458c5aede1135426a9f19f8584a7
Reviewed-on: https://go-review.googlesource.com/c/76312
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/ssa/export_test.go

@@ -163,7 +163,6 @@ func init() {
 	}
 	types.Dowidth = func(t *types.Type) {}
 
-	types.Tptr = types.TPTR64
 	for _, typ := range [...]struct {
 		width int64
 		et    types.EType