refactor(tvix/eval): Compile OpAssert using conditional jumps

In order to behave nicely with tryEval, asserts need to leave the
instruction pointer in a reasonable place even if they fail - whereas
with the previous implementation catching a failed assert would still
end up running the op for the *body* of the assert. With this change, we
compile asserts much more like an `if` expression with conditional jumps
rather than having an OpAssert op.

Change-Id: I1b266c3be90185c84000da6b1995ac3e6fd5471b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6925
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/opcode.rs

@@ -121,8 +121,8 @@ pub enum OpCode {
     /// Close scopes while leaving their expression value around.
     OpCloseScope(Count), // number of locals to pop
 
-    /// Asserts stack top is a boolean, and true.
-    OpAssert,
+    /// Return an error indicating that an `assert` failed
+    OpAssertFail,
 
     // Lambdas & closures
     OpCall,