Update load/store utilities for 64-bit backends

Update load/store utilities for 64-bit backends

This CL replaces the typical use of LoadWord/StoreWord
utilities (which, in practice, were 32-bit load/store) in
favor of a new set that make the size explicit.  We now have:

   LoadWordDisp/StoreWordDisp:
    32 or 64 depending on target.  Load or store the natural
    word size.  Expect this to be used infrequently - generally
    when we know we're dealing with a native pointer or flushed
    register not holding a Dalvik value (Dalvik values will flush
    to home location sizes based on Dalvik, rather than the target).

   Load32Disp/Store32Disp:
     Load or store 32 bits, regardless of target.

   Load64Disp/Store64Disp:
     Load or store 64 bits, regardless of target.

   LoadRefDisp:
     Load a 32-bit compressed reference, and expand it to the
     natural word size in the target register.

   StoreRefDisp:
     Compress a reference held in a register of the natural word
     size and store it as a 32-bit compressed reference.

Change-Id: I50fcbc8684476abd9527777ee7c152c61ba41c6f