[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.
Reviewed By: arsenm
Differential Revision: https://reviews.llvm.org/D53937