Fixed rounding issue in spirv_assembly tests

Fixed rounding issue in spirv_assembly tests

The test assumes that the host system and the device use the same
rounding mode. When this is not the case, the derived rounding errors
make the test fail unexpectedly. By rounding the input values down we
avoid differences in the test output caused by the rounding modes in
use.

Affects:

dEQP-VK.spirv_assembly.instruction.compute.loop_control.dependency_infinite

Components: Vulkan

VK-GL-CTS issue: 1097

Change-Id: I3678e38cbe5c6d57cecef91e7398df0d04763851