intel/fs: Assert if lower_source_modifiers converts 32x16 to 32x32 multiplication

intel/fs: Assert if lower_source_modifiers converts 32x16 to 32x32 multiplication

Lowering source modifiers will convert a 16bit source to a 32bit
value.  In the case of integer multiplication, this will reverse
previous lowering performed by lower_mul_dword_inst.

Assert to prevent an illegal DxD operation (and GPU hang).

Reviewed-by: Francisco Jerez <currojerez@riseup.net>