Eliminate the explicit volatile fields in LoadInst and StoreInst. This shrinks

LoadInst from 60 -> 56 bytes and StoreInst from 76 -> 72 bytes.

Note however, that this doesn't actually save any memory on common systems
where 'malloc' returns 8-byte aligned memory, as the saved space is replaced
by useless alignment padding.  :(

llvm-svn: 20046