For the ARM integrated assembler add checking of the
alignments on vld/vst instructions.  And report errors for
alignments that are not supported.

While this is a large diff and an big test case, the changes
are very straight forward.  But pretty much had to touch
all vld/vst instructions changing the addrmode to one of the
new ones that where added will do the proper checking for
the specific instruction.

FYI, re-committing this with a tweak so MemoryOp's default
constructor is trivial and will work with MSVC 2012. Thanks
to Reid Kleckner and Jim Grosbach for help with the tweak.

rdar://11312406