spirv: fix bug when OpSpecConstantOp calls a conversion

spirv: fix bug when OpSpecConstantOp calls a conversion

In that case, nir_eval_const_opcode() will evaluate the conversion
but as it was using destination's bit_size, the resulting
value was just a cast of the source constant value. By passing the
source's bit size, it does the conversion properly.

Fixes:

dEQP-VK.spirv_assembly.instruction.*.opspecconstantop.*convert*

v2:
- Remove invalid conversion op cases.

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>