R600: Use i24 optimized path for SREM

v2: add tests
    rename LowerSDIV24 to LowerSDIVREM24
    handle the rem part in this function

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
R600: Remove unused code.

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
R600: Use optimized 24bit path in udivrem

v2: drop enum keyword
    use correct extension mode
    don't bother computing the sign in unsinged case

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>