[dev.ssa] cmd/compile: add GOSSAFUNC and GOSSAPKG

These temporary environment variables make it
possible to enable using SSA-generated code
for a particular function or package without
having to rebuild the compiler.

This makes it possible to start bulk testing
SSA generated code.

First, bump up the default stack size
(_StackMin in runtime/stack2.go) to something
large like 32768, because without stackmaps
we can't grow stacks.

Then run something like:

for pkg in `go list std`
do
  GOGC=off GOSSAPKG=`basename $pkg` go test -a $pkg
done

When a test fails, you can re-run those tests,
selectively enabling one function after another,
until you find the one that is causing trouble.

Doing this right now yields some interesting results:

* There are several packages for which we generate
  some code and whose tests pass. Yay!

* We can generate code for encoding/base64, but
  tests there fail, so there's a bug to fix.

* Attempting to build the runtime yields a panic during codegen:
  panic: interface conversion: ssa.Location is nil, not *ssa.LocalSlot

* The top unimplemented codegen items are (simplified):
  59 genValue not implemented: REPMOVSB
  18 genValue not implemented: REPSTOSQ
  14 genValue not implemented: SUBQ
   9 branch not implemented: If v -> b b. Control: XORQconst <bool> [1]
   8 genValue not implemented: MOVQstoreidx8
   4 branch not implemented: If v -> b b. Control: SETG <bool>
   3 branch not implemented: If v -> b b. Control: SETLE <bool>
   2 load flags not implemented: LoadReg8 <flags>
   2 genValue not implemented: InvertFlags <flags>
   1 store flags not implemented: StoreReg8 <flags>
   1 branch not implemented: If v -> b b. Control: SETGE <bool>

Change-Id: Ib64809ac0c917e25bcae27829ae634c70d290c7f
Reviewed-on: https://go-review.googlesource.com/12547
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -60,6 +60,8 @@ func NewConfig(arch string, fe Frontend) *Config {
 	return c
 }
 
+func (c *Config) Frontend() Frontend { return c.fe }
+
 // NewFunc returns a new, empty function object
 func (c *Config) NewFunc() *Func {
 	// TODO(khr): should this function take name, type, etc. as arguments?