cmd/compile/internal: stop lowering OpConvert on riscv64
Lowering for OpConvert was removed for all architectures in CL#108496,
prior to the riscv64 port being upstreamed. Remove lowering of OpConvert
on riscv64, which brings it inline with all other architectures. This
results in 1,600+ instructions being removed from the riscv64 go binary.
Change-Id: Iaaf1f8b397875926604048b66ad8ac91a98c871e
Reviewed-on: https://go-review.googlesource.com/c/go/+/533335
Run-TryBot: Joel Sing <joel@sing.id.au> Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Michael Pratt <mpratt@google.com>