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>

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

@@ -1002,6 +1002,20 @@ var linuxAMD64Tests = []*asmTest{
 		}`,
 		pos: []string{"\tCMPL\t[A-Z]"},
 	},
+	{
+		fn: `
+		func $(a,b [3]int16) bool {
+		    return a == b
+		}`,
+		pos: []string{"\tCMPL\t[A-Z]"},
+	},
+	{
+		fn: `
+		func $(a,b [12]int8) bool {
+		    return a == b
+		}`,
+		pos: []string{"\tCMPQ\t[A-Z]", "\tCMPL\t[A-Z]"},
+	},
 	{
 		fn: `
 		func f70(a,b [15]byte) bool {