check and fix If clause of translateStatement

[ivoysey]

There are two todos in codegen that mark a site of a likely bug. I think that there isn't an assignment back to the temporary variable at the very end of the code we generate, which means that the value gets computed but then dropped on the floor. This bug existed in IfThenElse as well, and I fixed it there, but I didn't look at these two sites closely enough to make a test that breaks and know for sure:

https://github.com/mcoblenz/Obsidian/blob/test_output/src/main/scala/edu/cmu/cs/obsidian/codegen/CodeGenYul.scala#L319

https://github.com/mcoblenz/Obsidian/blob/test_output/src/main/scala/edu/cmu/cs/obsidian/codegen/CodeGenYul.scala#L325

• try to write at test that breaks the output
• fix it in an analogous way to IfThenElse if it is indeed broken
• look for other cases that have the same bug still
• write a helper function that has the right pattern in it once and for all and use that instead of doing it by hand in each case.