fix(tvix/eval): ensure that root stack slot actually exists

Instead of using a sentinel LocalIdx which potentially points to a
value in the locals stack that does not actually exist, set up an
initial uninitialised phantom value representing the result of the
root expression.

Change-Id: I82ea774daab83168020a3850bed57d35ab25c7df
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6424
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/compiler/scope.rs

@@ -128,10 +128,6 @@ pub struct Upvalue {
 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd)]
 pub struct LocalIdx(usize);
 
-impl LocalIdx {
-    pub const ZERO: LocalIdx = LocalIdx(0);
-}
-
 /// Represents a scope known during compilation, which can be resolved
 /// directly to stack indices.
 ///