[AMDGPU] should expand ROTL i16 to shifts.

Instruction combining pass turns library rotl implementation to llvm.fshl.i16.
In the selection dag the intrinsic is turned to ISD::ROTL node that cannot be selected.
Need to expand it to shifts again.

Reviewed By: rampitec, arsenm

Differential Revision: https://reviews.llvm.org/D87618