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This change is all about leveraging the gc bitmap generation
that is already done by the current compiler.  We rearrange how
stack allocation is done so that we generate a variable declaration
for each spill.  We also reorganize how args/locals are recorded
during SSA.  Then we can use the existing allocauto/defframe to
allocate the stack frame and liveness to make the gc bitmaps.

With this change, stack copying works correctly and we no longer
need hacks in runtime/stack*.go to make tests work.  GC is close
to working, it just needs write barriers.

Change-Id: I990fb4e3fbe98850c6be35c3185a1c85d9e1a6ba
Reviewed-on: https://go-review.googlesource.com/13894

Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>