cmd/compile: fix naming of decomposed structs

cmd/compile: fix naming of decomposed structs

When a struct is SSAable, we will name its component parts
by their field names.  For example,
type T struct {
     a, b, c int
}
If we ever need to spill a variable x of type T, we will
spill its individual components to variables named x.a, x.b,
and x.c.

Change-Id: I857286ff1f2597f2c4bbd7b4c0b936386fb37131
Reviewed-on: https://go-review.googlesource.com/21389
Reviewed-by: David Chase <drchase@google.com>