LoongArch (except for the extremely reduced LA32 Primary subset) has
dedicated beqz/bnez instructions as alternative encodings for beq/bne
with one of the source registers being R0, that allow the offset field
to occupy 5 more bits, giving 21 bits in total (equal to the FP
branches). Make use of them instead of beq/bne if one source operand is
omitted in asm, or if one of the registers being compared is R0.

Multiple go1 benchmark runs indicate the change is not perf-sensitive.

Change-Id: If6267623c82092e81d75578091fb4e013658b9f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/478377


Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Run-TryBot: Ben Shi <powerman1st@163.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Wayne Zuo <wdvxdr@golangcn.org>