[AMDGPU] Support idot2 pattern.

Summary: Transform add (mul ((i32)S0.x, (i32)S1.x),

         add( mul ((i32)S0.y, (i32)S1.y), (i32)S3) => i/udot2((v2i16)S0, (v2i16)S1, (i32)S3)

Author: FarhanaAleen

Reviewed By: arsenm

Subscribers: llvm-commits, AMDGPU

Differential Revision: https://reviews.llvm.org/D50024

llvm-svn: 340295