[HWLoops] Stop converting to a while loop when it would be unsafe to

There were cases where a do-while loop would be converted to a while
loop before finding out that it would be unsafe to expand the SCEV in
this situation and then bailing out of hardware loop conversion.

This patch checks if it would be unsafe to expand the SCEV and if so stops converting the do-while into a while, allowing conversion to a hardware loop.

Differential Revision: https://reviews.llvm.org/D83953