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/func.go

@@ -11,8 +11,9 @@ import (
 	"strings"
 )
 
-// A Func represents a Go func declaration (or function literal) and
-// its body. This package compiles each Func independently.
+// A Func represents a Go func declaration (or function literal) and its body.
+// This package compiles each Func independently.
+// Funcs are single-use; a new Func must be created for every compiled function.
 type Func struct {
 	Config *Config  // architecture information
 	pass   *pass    // current pass information (name, options, etc.)