LoongArch: Fix mode size comparision in loongarch_expand_conditional_move

LoongArch: Fix mode size comparision in loongarch_expand_conditional_move

We were comparing a mode size with word_mode, but word_mode is an enum
value thus this does not really make any sense.  (Un)luckily E_DImode
happens to be 8 so this seemed to work, but let's make it correct so it
won't blow up when we add LA32 support or add another machine mode...

gcc/ChangeLog:

	* config/loongarch/loongarch.cc
	(loongarch_expand_conditional_move): Compare mode size with
	UNITS_PER_WORD instead of word_mode.