Add instruction encoding for the Neon VMOV immediate instruction. This changes

the machine instruction representation of the immediate value to be encoded
into an integer with similar fields as the actual VMOV instruction.  This makes
things easier for the disassembler, since it can just stuff the bits into the
immediate operand, but harder for the asm printer since it has to decode the
value to be printed.  Testcase for the encoding will follow later when MC has
more support for ARM.

llvm-svn: 105836