cmd/compile: add mul by ±2ⁿ code-generation tests for arm/arm64

This change adds code generation tests for multiplication by ±2ⁿ for
arm and arm64, in preparation for a future CL which will remove the
relevant architecture-specific SSA rules (the reduction is already
performed by rules in generic.rules added in CL 36323).

Change-Id: Iebdd5c3bb2fc632c85888569ff0c49f78569a862
Reviewed-on: https://go-review.googlesource.com/75752Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -1816,6 +1816,26 @@ var linuxS390XTests = []*asmTest{
 }
 
 var linuxARMTests = []*asmTest{
+	// multiplication by powers of two
+	{
+		fn: `
+		func $(n int) int {
+			return 16*n
+		}
+		`,
+		pos: []string{"\tSLL\t[$]4"},
+		neg: []string{"\tMUL\t"},
+	},
+	{
+		fn: `
+		func $(n int) int {
+			return -32*n
+		}
+		`,
+		pos: []string{"\tSLL\t[$]5"},
+		neg: []string{"\tMUL\t"},
+	},
+
 	{
 		fn: `
 		func f0(x uint32) uint32 {
@@ -1943,6 +1963,26 @@ var linuxARMTests = []*asmTest{
 }
 
 var linuxARM64Tests = []*asmTest{
+	// multiplication by powers of two
+	{
+		fn: `
+		func $(n int) int {
+			return 64*n
+		}
+		`,
+		pos: []string{"\tLSL\t[$]6"},
+		neg: []string{"\tMUL\t"},
+	},
+	{
+		fn: `
+		func $(n int) int {
+			return -128*n
+		}
+		`,
+		pos: []string{"\tLSL\t[$]7"},
+		neg: []string{"\tMUL\t"},
+	},
+
 	{
 		fn: `
 		func f0(x uint64) uint64 {