[AMDGPU] Handle the idot8 pattern generated by FE.

Summary: Different variants of idot8 codegen dag patterns are not generated by llvm-tablegen due to a huge
         increase in the compile time. Support the pattern that clang FE generates after reordering the
         additions in integer-dot8 source language pattern.

Author: FarhanaAleen

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D53937

llvm-svn: 345902