[AMDGPU] Match udot8 pattern

Summary: D.u32 = S0.u4[0] * S1.u4[0] +

         S0.u4[1] * S1.u4[1] +
         S0.u4[2] * S1.u4[2] +
         S0.u4[3] * S1.u4[3] +
         S0.u4[4] * S1.u4[4] +
         S0.u4[5] * S1.u4[5] +
         S0.u4[6] * S1.u4[6] +
         S0.u4[7] * S1.u4[7] +
         S2.u32

Author: FarhanaAleen

Reviewed By: arsenm, nhaehnle

Differential Revision: https://reviews.llvm.org/D51947

llvm-svn: 342497