Add a 'test legaliser' filetest command.

This test command sends each function through legalize_function() and
then filecheck.

cranelift/filetests/isa/riscv/encoding.cton

@@ -0,0 +1,19 @@
+test legalizer
+isa riscv supports_m=1
+
+function int32(i32, i32) {
+ebb0(v1: i32, v2: i32):
+    v10 = iadd v1, v2
+    ; check: [R#0c]
+    ; sameln: $v10 = iadd
+
+    v11 = isub v1, v2
+    ; check: [R#200c]
+    ; sameln: $v11 = isub
+
+    v12 = imul v1, v2
+    ; check: [R#10c]
+    ; sameln: $v12 = imul
+
+    return
+}