Fix two infinite loops in codegen

[smoelius]

This PR fixes infinite lops in the reaching definitions and dead storage code.

The two added test files are based on ones from Solidity:

• https://github.com/ethereum/solidity/blob/develop/test/libsolidity/smtCheckerTests/loops/do_while_break_2.sol
• https://github.com/ethereum/solidity/blob/develop/test/libsolidity/smtCheckerTests/loops/do_while_break_2_fail.sol

If you add dbg!(block_no) between the next two lines and run Solang on either of the test files, you will find that a block is visited repeatedly:

solang/src/codegen/reaching_definitions.rs

Lines 56 to 57 in 279ee91

	while let Some(block_no) = blocks_todo.iter().next() {
	let block_no = *block_no;

If you fix that infinite loop and add dbg!(block_no) between the next two lines, you will again find that a block is visited repeatedly:

solang/src/codegen/dead_storage.rs

Lines 109 to 110 in 279ee91

	while let Some(block_no) = blocks_todo.iter().next() {
	let block_no = *block_no;

In both cases, the problem appears to be that edges are added to the blocks_todo queue unconditionally.

My understanding of these algorithms is that they try to iterate to a fixpoint. Hence, I changed each loop so that:

• a flag records whether a state change has occurred
• at the end of each loop iteration, the edge is added to the queue only if the flag is set

Furthermore, I analyzed each loop to try to determine what constitutes a state change. On this point, I invite scrutiny.

Nits are welcome on anything, though.

[seanyoung]

Totally makes sense. Thank you very much @smoelius

[smoelius]

Thank you, @seanyoung!