R600: Simplify LowerUDIVREM

optimizations can handle removing the Hi part operations.
The generated code is identical for R600, ~10% icount reduction for SI

v2: rebase

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Matt Arsenault <Matthew.Arsenault@amd.com>
llvm-svn: 226879