cmd/compile/internal/ssa: add tests for BSWAP on stores on AMD64

Commit 10f75748 (CL 32222) taught AMD64 backend to rewrite series of
byte loads or stores with corresponding shifts into a single long or
quad load or store + appropriate BSWAP. However it did not added test
for stores - only loads were tested.

Fix it.

NOTE Tests for indexed stores are not added because 10f75748 did not add
support for indexed stores - only indexed loads were handled then.

Change-Id: I48c867ebe7622ac8e691d43741feed1d40cca0d7
Reviewed-on: https://go-review.googlesource.com/34634Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -175,6 +175,14 @@ func f(b []byte, i int) uint64 {
 	},
 	{"amd64", "linux", `
 import "encoding/binary"
+func f(b []byte, v uint64) {
+	binary.BigEndian.PutUint64(b, v)
+}
+`,
+		[]string{"\tBSWAPQ\t"},
+	},
+	{"amd64", "linux", `
+import "encoding/binary"
 func f(b []byte) uint32 {
 	return binary.BigEndian.Uint32(b)
 }
@@ -186,6 +194,14 @@ import "encoding/binary"
 func f(b []byte, i int) uint32 {
 	return binary.BigEndian.Uint32(b[i:])
 }
+`,
+		[]string{"\tBSWAPL\t"},
+	},
+	{"amd64", "linux", `
+import "encoding/binary"
+func f(b []byte, v uint32) {
+	binary.BigEndian.PutUint32(b, v)
+}
 `,
 		[]string{"\tBSWAPL\t"},
 	},