[dev.ssa] cmd/internal/gc: convert standard IR into SSA.

[dev.ssa] cmd/internal/gc: convert standard IR into SSA.

Hook into the current compiler to convert the existing
IR (after walk) into SSA.  Any function ending in "_ssa"
will take this path.  The resulting assembly is printed
and then discarded.

Use gc.Type directly in ssa instead of a wrapper for go types.
It makes the IR->SSA rewrite a lot simpler.

Only a few opcodes are implemented in this change.  It is
enough to compile simple examples like
    func f(p *int) int { return *p }
    func g(a []int, i int) int { return a[i] }

Change-Id: I5e18841b752a83ca0519aa1b2d36ef02ce1de6f9
Reviewed-on: https://go-review.googlesource.com/8971
Reviewed-by: Alan Donovan <adonovan@google.com>