fix(tvix/eval): fix branching on catchable defaults (b/343)

This commit adds Opcode::OpJumpIfCatchable, which can be inserted
ahead of most VM operations which expect a boolean on the stack, in
order to handle catchables in branching position properly.

Other than remembering to patch the jump, no other changes should be
required.

This commit also fixes b/343 by emitting this new opcode when
compiling if-then-else.  There are probably other places where we
need to do the same thing.

Change-Id: I48de3010014c0bbeba15d34fc0d4800e0bb5a1ef
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10288
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: Adam Joseph <adam@westernsemico.com>

tvix/eval/src/value/mod.rs

@@ -639,6 +639,7 @@ impl Value {
     gen_is!(is_number, Value::Integer(_) | Value::Float(_));
     gen_is!(is_bool, Value::Bool(_));
     gen_is!(is_attrs, Value::Attrs(_));
+    gen_is!(is_catchable, Value::Catchable(_));
 
     /// Compare `self` against other using (fallible) Nix ordering semantics.
     ///