[tilegx] Optimize truncate(setcc).

  For TileGX, setcc, ie.cmpXXX, always write 64bit registers, so the
the high part of the reg is zeroed automatically, there is no need
for an explict truncation.
[tilegx] Reset setcc result to i32.

  * reset setcc result type to i32, to be consistent with other
    type promotion.

  * optimize (i64 zero_extend (i32 setcc)). remove zero_extend as
    TileGX's setcc pattern always operate on 64bit reg.