r300/compiler: Handle loops in deadcode analysis.

r300/compiler: Handle loops in deadcode analysis.

This also allows us to split the loop emulation into two phases.  A
tranformation phase which either unrolls loops or prepares them to be
emulated, and the emulation phase which unrolls remaining loops until the
instruction limit is reached.  The second phase is completed after the
deadcode analysis in order to get a more accurate count of the number of
instructions in the body of loops.