Change the 'isStore' inferrer to look for 'SDNPMayStore'

instead of "ISD::STORE".  This allows us to mark target-specific dag
nodes as storing (such as ppc byteswap stores).  This allows us to remove
more explicit isStore flags from the .td files.

Finally, add a warning for when a .td file contains an explicit 
isStore and tblgen is able to infer it.

llvm-svn: 45654