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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@45654 91177308-0d34-0410-b5e6-96231b3b80d8