radv: ensure export arguments are always float

So that the signature is correct and consistent, the inputs to a export
intrinsic should always be 32-bit floats.

This and the previous commit fixes a large amount crashes from
dEQP-VK.spirv_assembly.instruction.graphics.16bit_storage.input_output_int_*
tests

Fixes: b722b29f ('radv: add support for 16bit input/output')
Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit 0ca550e0)