cmd/compile/internal/ir: drop Inl.Body

We don't actually depend on Inl.Body anywhere, except it implicitly
serves to indicate whether Inl.Dcl has been populated. So replace it
with a boolean so we don't need to keep a useless copy of every
inlinable function body in memory.

While here, also add some Fatalfs to make sure there are no unused
local variables. The unified frontend now omits unreachable code
during export data writing, so there shouldn't be unused local
variables.

Also, since unified IR uses the same code/data to construct the
original function as inlined and/or imported functions, the Dcl list
should always be the same, which addresses the real root issue (i.e.,
that export/import could skew the Dcl lists).

Change-Id: I6e3435f3a0352f6efbae787344006efac1891e84
Reviewed-on: https://go-review.googlesource.com/c/go/+/523315
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/compile/internal/noder/linker.go

@@ -233,7 +233,7 @@ func (l *linker) exportBody(obj *ir.Name, local bool) {
 	//
 	// TODO(mdempsky): Reimplement the reachable method crawling logic
 	// from typecheck/crawler.go.
-	exportBody := local || fn.Inl.Body != nil
+	exportBody := local || fn.Inl.HaveDcl
 	if !exportBody {
 		return
 	}