cmd/compile/internal/x86: avoid CX in a couple of places in the int64 code

I want to use CX as a scratch register in position independent code and these
uses are easy to remove.

Change-Id: I9e3cb470d7f0000d85786c30bd769d9ec86d532a
Reviewed-on: https://go-review.googlesource.com/16382


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>