optimize drutil_insert_get_mem_addr() for cases where lea not needed

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on April 12, 2012 10:24:36_

one case from timur:

For
...
mov (%eax) -> %eax
...
original instruction
drutil_insert_get_mem_addr(..., REG_XAX, REG_XCX)
produces
...
[spill
lea (%eax) -> %eax
mov (%eax) -> %eax
...
Is there any reason to create `lea` at all here?
Maybe it could just avoid emitting anything?

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=729_