Remove support for entity variables in filecheck.

Now that the parser doesn't renumber indices, there's no need for entity
variables like $v0.

cranelift/filetests/regalloc/intel-regres.cton

@@ -19,12 +19,12 @@ ebb0(v0: i32):
     jump ebb2(v3, v2, v0)
 
 ebb2(v4: i32, v5: i32, v7: i32):
-    ; check: $ebb2
+    ; check: ebb2
     v6 = iadd v4, v5
     v8 = iconst.i32 -1
     ; v7 is killed here and v9 gets the same register.
     v9 = iadd v7, v8
-    ; check: $v9 = iadd $v7, $v8
+    ; check: v9 = iadd v7, v8
     ; Here v9 the brnz control appears to interfere with v9 the EBB argument,
     ; so divert_fixed_input_conflicts() calls add_var(v9), which is ok. The
     ; add_var sanity checks got confused when no fixed assignment could be
@@ -32,7 +32,7 @@ ebb2(v4: i32, v5: i32, v7: i32):
     ;
     ; We should be able to handle this situation without making copies of v9.
     brnz v9, ebb2(v5, v6, v9)
-    ; check: brnz $v9, $ebb2($V, $V, $v9)
+    ; check: brnz v9, ebb2($V, $V, v9)
     jump ebb3
 
 ebb3: