LoongArch: Simplify "BEQ/BNE foo, zero" with BEQZ/BNEZ

While B{EQ,NE}Z and B{EQ,NE} are different instructions, and the vastly
expanded range for branch destination does not really matter in the few
cases touched, use the B{EQ,NE}Z where possible for shorter lines and
better consistency (e.g. some places used "BEQ foo, zero", while some
used "BEQ zero, foo").

Signed-off-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>