cmd/internal/obj/mips: use obj.REG_NONE rather than 0
Instead of comparing register values to 0, use obj.REG_NONE to improve
readability. No functional change.
Change-Id: I848f7ab7e27fe7e5e9fe2caf9cce47e372c6cd6c
Reviewed-on: https://go-review.googlesource.com/c/go/+/514096
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Run-TryBot: Joel Sing <joel@sing.id.au> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Rong Zhang <rongrong@oss.cipunited.com>
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