cmd/compile: avoid generating duplicated in-register Arg

In expand_calls, when rewriting OpArg to OpArgIntReg/OpArgFloatReg,
avoid generating duplicates. Otherwise it will confuse the
register allocator: it would think the second occurance clobbers
the first's register, causing it to generate copies, which may
clobber other args.

Change-Id: I4f1dc0519afb77500eae1c0e6ac8745e51f7aa4e
Reviewed-on: https://go-review.googlesource.com/c/go/+/306029


Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: David Chase <drchase@google.com>