intel/compiler: handle 16-bit to 64-bit conversions in BSW platforms

intel/compiler: handle 16-bit to 64-bit conversions in BSW platforms

These are subject to the general restriction that anything that is converted
to 64-bit needs to be aligned to 64-bit.  We had this already in place for
32-bit to 64-bit conversions, so this patch generalizes the implementation
to take effect on any conversion to 64-bit from a source smaller than
64-bit.

Fixes assembly validation errors in the following CTS tests in BSW:
dEQP-VK.spirv_assembly.instruction.compute.sconvert.int16_to_int64
dEQP-VK.spirv_assembly.instruction.compute.uconvert.uint16_to_uint64
dEQP-VK.spirv_assembly.instruction.compute.sconvert.int16_to_uint64

Tested-by: Mark Janes <mark.a.janes@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>