[X86] Be more structured about how we capture the register number when it is...

[X86] Be more structured about how we capture the register number when it is encoded in bits 7:4 of the immediate.

For some instructions the register is not the last operand and the immediate handling had to detect this and hardcode the index to find it. It also required CurOp to be pointing at the last operand handled in the Form switch whereas for any instruction it would be pointing at the next operand.

Now we just capture the value in the Form switch when we know exactly where it is and the CurOp pointer can behave normally.

llvm-svn: 262462