[dev.ssa] cmd/compile: fix spill sizes

[dev.ssa] cmd/compile: fix spill sizes

In code that does:

    var x, z int32
    var y int64
    z = phi(x, int32(y))

We silently drop the int32 cast because truncation is a no-op.
The phi operation needs to make sure it uses the size of the
phi, not the size of its arguments, when generating spills.

Change-Id: I1f7baf44f019256977a46fdd3dad1972be209042
Reviewed-on: https://go-review.googlesource.com/18390
Reviewed-by: David Chase <drchase@google.com>