cmd/compile: recognize bit test patterns on amd64

Updates #18943

Change-Id: If3080d6133bb6d2710b57294da24c90251ab4e08
Reviewed-on: https://go-review.googlesource.com/36329
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -504,6 +504,45 @@ var linuxAMD64Tests = []*asmTest{
 		`,
 		[]string{"\"abc\""},
 	},
+	// Bit test ops on amd64, issue 18943.
+	{
+		`
+		func f37(a, b uint64) int {
+			if a&(1<<(b&63)) != 0 {
+				return 1
+			}
+			return -1
+		}
+		`,
+		[]string{"\tBTQ\t"},
+	},
+	{
+		`
+		func f38(a, b uint64) bool {
+			return a&(1<<(b&63)) != 0
+		}
+		`,
+		[]string{"\tBTQ\t"},
+	},
+	{
+		`
+		func f39(a uint64) int {
+			if a&(1<<60) != 0 {
+				return 1
+			}
+			return -1
+		}
+		`,
+		[]string{"\tBTQ\t\\$60"},
+	},
+	{
+		`
+		func f40(a uint64) bool {
+			return a&(1<<60) != 0
+		}
+		`,
+		[]string{"\tBTQ\t\\$60"},
+	},
 }
 
 var linux386Tests = []*asmTest{