Permit not flushing operands for FP16 DenormFlushToZero add, dot, vmulm

Permit not flushing operands for FP16 DenormFlushToZero add, dot, vmulm

Where `d` is a denormal value such that `2 * d` is not denormal:

- The F16 OpFAdd tests assumed (d + d) is zero when DenormFlushToZero is
  used. This is not true if the operands (d) are not flushed to zero by
  the implementation, which is allowed by the Vulkan specification.
- The F16 OpDot test assumed dot(vec2(d, d), vec2(1, 1)) is zero. Since
  that is equivalent to (d + d), it has the same problem as OpFAdd.
- The F16 OpVectorTimesMatrix test made the same assumption as the OpDot
  test but for both columns of a 2-by-2 matrix, with the same problem.

These tests are changed to accept `2 * d` as a valid result alongside
zero, such that they will pass both on implementations which flush
operands to zero and on implementations which only flush results to
zero.

Modified tests:

    dEQP-VK.spirv_assembly.instruction.*.float_controls.fp16.input_args.denorm_*flush_to_zero*
    dEQP-VK.spirv_assembly.instruction.compute.float_controls.independence_settings.denorm_ind_*_fp16_flush_*

VK-GL-CTS Issue: 2644

Components: Vulkan

Change-Id: I0ef4a57915e3bdab100f55f52d21913f87af4927