cmd/compile: avoid extending floating point comparision on riscv64

cmd/compile: avoid extending floating point comparision on riscv64

The result of these operations are already extended.

Change-Id: Ifc8ba362dda7035d8fd0d40046a96f61d3082877
Reviewed-on: https://go-review.googlesource.com/c/go/+/426260
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>