[llvm-reduce] Do not crash when accessing landingpads of invokes.

Unconditionally removing landing pads results in invalid IR,
if there is a different `invoke` that uses it. Update the code
to only remove the landing pad if the current invoke is the only
user. Also carefully avoid creating plain branches to bbs with
landing pads we couldn't remove.

Reviewed By: arsenm, aeubanks

Differential Revision: https://reviews.llvm.org/D138072