Fix internal representation of fp80 to be the

Fix internal representation of fp80 to be the
same as a normal i80 {low64, high16} rather
than its own {high64, low16}.  A depressing number
of places know about this; I think I got them all.
Bitcode readers and writers convert back to the old
form to avoid breaking compatibility.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@67562 91177308-0d34-0410-b5e6-96231b3b80d8