[dev.ssa] cmd/compile: introduce storeconst ops

[dev.ssa] cmd/compile: introduce storeconst ops

Introduce opcodes that store a constant value.
AuxInt now needs to hold both the value to be stored and the
constant offset at which to store it.  Introduce a StoreConst
type to help encode/decode these parts to/from an AuxInt.

Change-Id: I1631883abe035cff4b16368683e1eb3d2ccb674d
Reviewed-on: https://go-review.googlesource.com/16170
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>