VFP instructions use it for loading some constants, so implement that
handling.
Not thrilled with adding a member to MCOperand, but not sure there's much of
a better option that's not pretty fragile (like putting a double in the
union instead and just assuming that's good enough). Suggestions welcome...
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@113996 91177308-0d34-0410-b5e6-96231b3b80d8
26edbcb8d5