[AMDGPU] [AMDGPU] Support a fdot2 pattern.

Summary: Optimize fma((float)S0.x, (float)S1.x fma((float)S0.y, (float)S1.y, z))
                   -> fdot2((v2f16)S0, (v2f16)S1, (float)z)

Author: FarhanaAleen

Reviewed By: rampitec, b-sumner

Subscribers: AMDGPU

Differential Revision: https://reviews.llvm.org/D49146

llvm-svn: 337198