[dev.ssa] cmd/compile: Allow structs to be SSAd

[dev.ssa] cmd/compile: Allow structs to be SSAd

Break small structs up into their components so they
can be registerized.

Change StructSelect to use field indexes instead of
field offsets, as field offsets aren't unique in the
presence of zero-sized fields.

Change-Id: I2f1dc89f7fa58e1cf58aa1a32b238959d53f62e4
Reviewed-on: https://go-review.googlesource.com/18570
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>