[optimization] Skip receipt appending during predicate execution

[Voxelot]

Receipts aren't available from predicates, however some opcodes like RET are allowed in a predicate context and will incur extra costs due to receipt merkleization (the result of which is just thrown away).

While predicates are unlikely to generate a significant number of receipts, removing this additional step would improve performance.

[xgreenx]

It as optimized during #678