[dev.regabi] cmd/compile: prep refactoring for switching to go/constant

This CL replaces gc.Ctype (along with its CTINT, etc. constants) with
constant.Kind; renames Val.Ctype to Val.Kind; and replaces a handful
of abstraction-violating patterns that can be readily expressed
differently.

The next commit will actually replace Val with constant.Value.

Passes toolstash-check.

[git-generate]
cd src/cmd/compile/internal/gc

sed -i 's/type Ctype uint8/type Ctype = constant.Kind/' const.go
goimports -w const.go

rf '
inline -rm Ctype
mv Val.Ctype Val.Kind

ex import "go/constant"; \
  CTxxx  -> constant.Unknown; \
  CTINT  -> constant.Int; \
  CTFLT  -> constant.Float; \
  CTCPLX -> constant.Complex; \
  CTBOOL -> constant.Bool; \
  CTSTR  -> constant.String

rm CTxxx CTINT CTFLT CTCPLX CTBOOL CTSTR

ex import "cmd/compile/internal/types"; \
  var t *types.Type; \
  var v, v2 Val; \
  v.U.(*Mpint).Cmp(maxintval[TINT]) > 0 -> doesoverflow(v, types.Types[TINT]); \
  v.U.(*Mpint).Cmp(v2.U.(*Mpint)) > 0 -> compareOp(v, OGT, v2); \
  maxintval[t.Etype].Cmp(maxintval[TUINT]) <= 0 -> t.Size() <= types.Types[TUINT].Size(); \
  maxintval[t.Etype].Cmp(maxintval[TUINT]) >  0 -> t.Size() >  types.Types[TUINT].Size();
'

go test cmd/compile -u

Change-Id: I6c22ec0597508845f88eee639a0d76cbaa66d08f
Reviewed-on: https://go-review.googlesource.com/c/go/+/272653
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>

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

@@ -13,6 +13,7 @@ import (
 	"crypto/sha256"
 	"encoding/json"
 	"fmt"
+	"go/constant"
 	"io"
 	"io/ioutil"
 	"os"
@@ -263,7 +264,7 @@ func dumpGlobalConst(n *Node) {
 	case TUINTPTR:
 		// ok
 	case TIDEAL:
-		if !Isconst(n, CTINT) {
+		if !Isconst(n, constant.Int) {
 			return
 		}
 		x := n.Val().U.(*Mpint)