[XRay][test] Fix xray-empty-firstmbb.mir and delete incorrect xray-empty-function.mir

xray-empty-firstmbb.mir does not test the intended code path. Change
xray-instruction-threshold to 0 to exercise the code path.

Delete xray-empty-function.mir . Empty MachineFunction does not work.
Various passes (e.g. MachineDominatorTree) assume the presence of an
entry block.

Author: MaskRay

llvm/test/CodeGen/X86/xray-empty-firstmbb.mir

@@ -4,7 +4,7 @@
 
 --- |
 
-  define i32 @foo() noinline uwtable "xray-instruction-threshold"="1" {
+  define i32 @foo() noinline uwtable "xray-instruction-threshold"="0" {
   entry:
     unreachable
   }