cmd/compile: correct a comment in uint64Tofloat
cmd/compile: correct a comment in uint64Tofloat

We only do z>>1 once (as it should be). Remove the extra one
in the comment.

Change-Id: I6fea05fe697de07e160bd686697377babcb3e4c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/347400
Trust: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Reviewed-by: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>

nawawi/go,cloudflare/go,golang/go,nawawi/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,CAFxX/go,golang/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go,golang/go,cloudflare/go,cloudflare/go,nawawi/go