Decompose breaks compound objects up into pieces that can be
operated on by the target architecture.  The decompose pass only
does phi ops, the rest is done by the rewrite rules in generic.rules.

Compound objects include strings,slices,interfaces,structs,arrays.

Arrays aren't decomposed because of indexing (we could support
constant indexes, but dynamic indexes can't be handled using SSA).
Structs will come in a subsequent CL.

TODO: after this pass we have lost the association between, e.g.,
a string's pointer and its size.  It would be nice if we could keep
that information around for debugging info somehow.

Change-Id: I6379ab962a7beef62297d0f68c421f22aa0a0901
Reviewed-on: https://go-review.googlesource.com/13683Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>