cmd/compile: use load and test instructions on s390x

The load and test instructions compare the given value
against zero and will produce a condition code indicating
one of the following scenarios:

0: Result is zero
1: Result is less than zero
2: Result is greater than zero
3: Result is not a number (NaN)

The instruction can be used to simplify floating point comparisons
against zero, which can enable further optimizations.

This CL also reduces the size of .text section of math.test binary by around
0.7 KB (in hexadecimal, from 1358f0 to 135620).

Change-Id: I33cb714f0c6feebac7a1c46dfcc735e7daceff9c
Reviewed-on: https://go-review.googlesource.com/c/go/+/209159


Reviewed-by: Michael Munday <mike.munday@ibm.com>
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>