cmd/compile: fix s390x unsigned comparison constant merging rules

On s390x unsigned integer comparisons with immediates require the immediate
to be an unsigned 32-bit integer. The rule was checking that the immediate
was a signed 32-bit integer.

This CL also adds a test for comparisons that could be turned into compare
with immediate or equivalent instructions (depending on architecture and
optimizations applied).

Fixes #19940.

Change-Id: Ifd6aa989fd3d50e282f7d30fec9db462c28422b1
Reviewed-on: https://go-review.googlesource.com/40433
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/gc/ssa_test.go

@@ -63,6 +63,9 @@ func TestArithmeticConst(t *testing.T) { runTest(t, "arithConst.go") }
 
 func TestChan(t *testing.T) { runTest(t, "chan.go") }
 
+// TestComparisonsConst tests results for comparison operations against constants.
+func TestComparisonsConst(t *testing.T) { runTest(t, "cmpConst.go") }
+
 func TestCompound(t *testing.T) { runTest(t, "compound.go") }
 
 func TestCtl(t *testing.T) { runTest(t, "ctl.go") }