R600: Implement custom SDIVREM.

Instead of separate SDIV/SREM. SDIV used UDIV which in turn used UDIVREM anyway.
SREM used SDIV(UDIV->UDIVREM)+MUL+SUB, using UDIVREM directly is more efficient.

v2: Don't use all caps names

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
llvm-svn: 211477