[Alignment][NFC] Use 5 bits to store Instructions Alignment

As per [MaxAlignmentExponent]{https://github.com/llvm/llvm-project/blob/b7338fb1a6a464472850211165391983d2c8fdf3/llvm/include/llvm/IR/Value.h#L688} alignment is not allowed to be more than 2^29.
Encoded as Log2, this means that storing alignment uses 5 bits.
This patch makes sure all instructions store their alignment in a consistent way, encoded as Log2 and using 5 bits.

Differential Revision: https://reviews.llvm.org/D83119