[dev.ssa] B[dev.ssa] cmd/internal/ssa: Cleanup & reorg

[dev.ssa] B[dev.ssa] cmd/internal/ssa: Cleanup & reorg

Rename ops like ADDCQ to ADDQconst, so it is clear what the base opcode is and what
the modifiers are.

Convert FP references to SP references once we know the frame size.  Related, compute
the frame size in the ssa package.

Do a bunch of small fixes.

Add a TODO list for people to peruse.

Change-Id: Ia6a3fe2bf57e5a2e5e883032e2a2a3fdd566c038
Reviewed-on: https://go-review.googlesource.com/10465
Reviewed-by: Alan Donovan <adonovan@google.com>