The implementations of those functions require the rounding target to be
power-of-two. It's better to add a debugging check to avoid misuse.
Besides, add a general verion of those three to accommadate non
power-of-two cases.
Also change the name to roundUp/roundDown/isAligned
Reviewed By: cferris, cryptoad
Differential Revision: https://reviews.llvm.org/D142658