[Yul Optimizer] Do not move loop invariants that consume a lot of gas

This is a weird one.

```
contract C0 {
function f0() external returns (bytes memory o1)
{
assembly
{
for { let i := 0 } lt(i, 1) { i := add(i, 1) }
{
if o1 { break }
o1 := keccak256(0, 12345678912)
}
}
}
}
// ====
// compileViaYul: also
// ----
// f0() -> 0x20, 0
```

fails via Yul.

To repro

```
$ cp test.sol test/libsolidity/semanticTests
$ isoltest -t semanticTests/test --optimize --show-messages
```

Start with the Yul optimizer behavior shown in the assembly example and reproduce it by placing the case in test/libsolidity/semanticTests. Run isoltest -t semanticTests/test --optimize --show-messages and compare the failing result with the expected f0() output. Done means the test passes via Yul without moving the costly loop-invariant operation.

Written by the indexing model from the issue text.