ARM fix encoding of VMOV.f32 and VMOV.f64 immediates.

Encode the immediate into its 8-bit form as part of isel rather than later,
which simplifies things for mapping the encoding bits, allows the removal
of the custom disassembler decoding hook, makes the operand printer trivial,
and prepares things more cleanly for handling these in the asm parser.

rdar://10211428

llvm-svn: 140834