cmd/compile: rename types.Rnd -> types.RoundUp

cmd/compile: rename types.Rnd -> types.RoundUp

Base on gri@'s suggestion in CL 308971. "Rnd" is a bit random.

Change-Id: I4aad8b7992b31dfd26d20b3c332bc6e1e90f67db
Reviewed-on: https://go-review.googlesource.com/c/go/+/422036
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
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