[dev.ssa] cmd/compile: used Bounded field to fix empty range loops
for i, v := range a {
}
Walk converts this to a regular for loop, like this:
for i := 0, p := &a[0]; i < len(a); i++, p++ {
v := *p
}
Unfortunately, &a[0] fails its bounds check when a is
the empty slice (or string). The old compiler gets around this
by marking &a[0] as Bounded, meaning "don't emit bounds checks
for this index op". This change makes SSA honor that same mark.
The SSA compiler hasn't implemented bounds check panics yet,
so the failed bounds check just causes the current routine
to return immediately.