[AMDGPU] Improve abs modifier usage

If a call to the llvm.fabs intrinsic has users in another reachable
BB, SelectionDAG will not apply the abs modifier to these users and
instead generate a v_and ..., 0x7fffffff instruction.
For fneg instructions, the issue is similar.
This patch implements `AMDGPUIselLowering::shouldSinkOperands`,
which allows CodegenPrepare to call `tryToSinkFreeOperands`.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D150347