cmd/compile: define roles for ssa.Func, ssa.Config, and ssa.Cache

The line between ssa.Func and ssa.Config has blurred.
Concurrent compilation in the backend will require more precision.
This CL lays out an (aspirational) organization.
The implementation will come in follow-up CLs,
once the organization is settled.

ssa.Config holds basic compiler configuration,
mostly arch-specific information.
It is configured once, early on, and is readonly,
so it is safe for concurrent use.

ssa.Func is a single-shot object used for
compiling a single Func. It is not concurrency-safe
and not re-usable.

ssa.Cache is a multi-use object used to avoid
expensive allocations during compilation.
Each ssa.Func is given an ssa.Cache to use.
ssa.Cache is not concurrency-safe.

Change-Id: Id02809b6f3541541cac6c27bbb598834888ce1cc
Reviewed-on: https://go-review.googlesource.com/38160
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -14,6 +14,9 @@ import (
 	"strings"
 )
 
+// A Config holds readonly compilation information.
+// It is created once, early during compilation,
+// and shared across all compilations.
 type Config struct {
 	arch            string                     // "amd64", etc.
 	IntSize         int64                      // 4 or 8