[release-branch.go1.9] cmd/compile: use unsigned loads for multi-element comparisons

When loading multiple elements of an array into a single register,
make sure we treat them as unsigned.  When treated as signed, the
upper bits might all be set, causing the shift-or combo to clobber
the values higher in the register.

Fixes #23719.

Change-Id: Ic87da03e9bd0fe2c60bb214b99f846e4e9446052
Reviewed-on: https://go-review.googlesource.com/92335


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>
Reviewed-on: https://go-review.googlesource.com/103115


Run-TryBot: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>