[AMDGPU] Match udot4 pattern.

Summary: D.u32 = S0.u8[0] * S1.u8[0] +
                 S0.u8[1] * S1.u8[1] +
                 S0.u8[2] * S1.u8[2] +
                 S0.u8[3] * S1.u8[3] + S2.u32

Author: FarhanaAleen

Reviewed By: arsenm

Subscribers: llvm-commits, AMDGPU

Differential Revision: https://reviews.llvm.org/D50921

llvm-svn: 340936