Simplify pinned-vreg API: don't require slice of all pinned vregs. (#28)

Simplify pinned-vreg API: don't require slice of all pinned vregs.

Previously, we kept a bool flag `is_pinned` in the `VRegData`, and we
required a `&[VReg]` of all pinned vregs to be provided by
`Function::pinned_vregs()`. This was (I think) done for convenience, but
it turns out not to really be necessary, as we can just query
`is_pinned_vreg` where needed (and in the likely implementation, e.g. in
Cranelift, this will be a `< NUM_PINNED_VREGS` check that can be
inlined). This adds convenience for the embedder (the main benefit), and
also reduces complexity, removes some state, and avoids some work
initializing the regalloc state for a run.

src/ion/moves.rs

@@ -209,11 +209,7 @@ impl<'a, F: Function> Env<'a, F> {
         for vreg in 0..self.vregs.len() {
             let vreg = VRegIndex::new(vreg);
 
-            let pinned_alloc = if self.vregs[vreg.index()].is_pinned {
-                self.func.is_pinned_vreg(self.vreg_regs[vreg.index()])
-            } else {
-                None
-            };
+            let pinned_alloc = self.func.is_pinned_vreg(self.vreg_regs[vreg.index()]);
 
             // For each range in each vreg, insert moves or
             // half-moves.  We also scan over `blockparam_ins` and