codegen-opt: fix stack alignment

previously, stack-alignment of, for example, a 12-byte
allocation would be computed to be 16-bytes during codegen
(because of gc-alignment rounding up),
but 8-bytes here (because of this code rounding down)

src/llvm-alloc-opt.cpp

@@ -715,10 +715,7 @@ bool AllocOpt::runOnFunction(Function &F)
  sz += align;
  }
  else if (sz > 1) {
- align = JL_SMALL_BYTE_ALIGNMENT;
- while (sz < align) {
- align = align / 2;
- }
+ align = llvm::MinAlign(JL_SMALL_BYTE_ALIGNMENT, llvm::NextPowerOf2(sz));
  }
  // No debug info for prolog instructions
  IRBuilder<> prolog_builder(&entry.front());