Fix warning in gcc-8

Fix warning in gcc-8

The code for setting up the output buffer was duplicated into each of
the cases for different input types, and the compiler doesn't understand
that DE_ASSERT marks an unreachable path. Changing to using standard
assert fixes the warning:

vktShaderFConvertTests.cpp:541:19: error: ‘outputBufferSizeInfo::memorySizeBytes’
may be used uninitialized in this function [-Werror=maybe-uninitialized]
   541 |  BufferSizeInfo   outputBufferSizeInfo;
       |                   ^~~~~~~~~~~~~~~~~~~~

Component: Vulkan
Affects: dEQP-VK.glsl.builtin.precision_fconvert.*

Change-Id: Icf6302166d8bce7c3ae1447973c895c34465a726