feat(tvix/eval): implement if/else expressions

These expressions use simple jumps to skip the correct expression
conditionally in the bytecode by advancing the instruction pointer.

Note that these expressions are already covered by a test behind the
`nix_tests` feature flag, but adding more is probably sensible.

Change-Id: Ibe0eba95d216321c883d3b6b5816e2ab6fe7eef1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6148
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/opcode.rs

@@ -12,6 +12,9 @@ pub enum OpCode {
     // Push a constant onto the stack.
     OpConstant(ConstantIdx),
 
+    // Discard a value from the stack.
+    OpPop,
+
     // Push a literal value.
     OpNull,
     OpTrue,
@@ -27,13 +30,17 @@ pub enum OpCode {
     OpMul,
     OpDiv,
 
-    // Logical binary operators
+    // Comparison operators
     OpEqual,
     OpLess,
     OpLessOrEq,
     OpMore,
     OpMoreOrEq,
 
+    // Logical operators & generic jumps
+    OpJump(usize),
+    OpJumpIfFalse(usize),
+
     // Attribute sets
     OpAttrs(usize),
     OpAttrPath(usize),