add query of whether an operand is implicit

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on May 07, 2011 21:11:50_

For optimizations or other transformations that want to substitute one
register for another, knowing whether an operand change can be encoded is
required. In the API, instr_encode() returns NULL if it fails to encode
(though it requires some scratch space since if successful it goes and
writes to a target address). This issue covers adding a more efficient
solution.

Being able to query whether an operand is implicit or not isn't trivial
to calculate efficiently for general level 4 instrs: the implementation
would have to go try and encode. It could cache the template like it
caches the bytes today and answer queries about each operand of the same
instr more efficiently than the client having to try to encode each one
I suppose, and for level 3 avoid the encode altogether.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=452_