fix stack corruption! Previously, 16-byte whole-FP-register stores were
being treated as needing only 8 bytes (though they were 16 byte aligned.)

This should fix a bunch of tests - anyone have any comments, though?

  - in Target.td , SpillSize and SpillAlignment seem dead - is this what
    Size and Alignment do now?
  - in CodeGenRegisters.h/CodeGenTarget.cpp , DeclaredSpillSize and
    DeclaredSpillAlignment seem dead.
  - there are a bunch of comments here and there that don't clearly
    distinguish between 'size' and 'spillsize' etc. hmm.

llvm-svn: 25644