arm64: Support bool constants

Copyright (c) 2020, Arm Limited.

cranelift/filetests/filetests/vcode/aarch64/constants.clif

@@ -1,6 +1,32 @@
 test vcode
 target aarch64
 
+function %f() -> b8 {
+block0:
+  v0 = bconst.b8 true
+  return v0
+}
+
+; check: stp fp, lr, [sp, #-16]!
+; nextln: mov fp, sp
+; nextln: movz x0, #1
+; nextln: mov sp, fp
+; nextln: ldp fp, lr, [sp], #16
+; nextln: ret
+
+function %f() -> b16 {
+block0:
+  v0 = bconst.b16 false
+  return v0
+}
+
+; check: stp fp, lr, [sp, #-16]!
+; nextln: mov fp, sp
+; nextln: movz x0, #0
+; nextln: mov sp, fp
+; nextln: ldp fp, lr, [sp], #16
+; nextln: ret
+
 function %f() -> i64 {
 block0:
   v0 = iconst.i64 0