cmd/vet: fix go vet on s390x assembly

Test adapted from the mips64 test.

Fixes #15454.

Change-Id: If890c2d18a4a03a08faaa2e674edd7223af60290
Reviewed-on: https://go-review.googlesource.com/22472
Run-TryBot: Michael Munday <munday@ca.ibm.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/vet/asmdecl.go

@@ -77,6 +77,7 @@ var (
 	asmArchMips64LE = asmArch{"mips64le", size88, false, "R29", true}
 	asmArchPpc64    = asmArch{"ppc64", size88, true, "R1", true}
 	asmArchPpc64LE  = asmArch{"ppc64le", size88, false, "R1", true}
+	asmArchS390X    = asmArch{"s390x", size88, true, "R15", true}
 
 	arches = []*asmArch{
 		&asmArch386,
@@ -88,6 +89,7 @@ var (
 		&asmArchMips64LE,
 		&asmArchPpc64,
 		&asmArchPpc64LE,
+		&asmArchS390X,
 	}
 )
 
@@ -648,6 +650,17 @@ func asmCheckVar(badf func(string, ...interface{}), fn *asmFunc, line, expr stri
 			case "MOVV", "MOVD":
 				src = 8
 			}
+		case "s390x":
+			switch op {
+			case "MOVB", "MOVBZ":
+				src = 1
+			case "MOVH", "MOVHZ":
+				src = 2
+			case "MOVW", "MOVWZ", "FMOVS":
+				src = 4
+			case "MOVD", "FMOVD":
+				src = 8
+			}
 		}
 	}
 	if dst == 0 {