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/isa/riscv/expand-i32.cton

@@ -14,9 +14,9 @@ ebb0(v1: i32, v2: i32):
     v3, v4 = iadd_cout v1, v2
     return v3, v4
 }
-; check: $v3 = iadd $v1, $v2
-; check: $v4 = icmp ult $v3, $v1
-; check: return $v3, $v4
+; check: v3 = iadd v1, v2
+; check: v4 = icmp ult v3, v1
+; check: return v3, v4
 
 ; Expanding illegal immediate constants.
 ; Note that at some point we'll probably expand the iconst as well.
@@ -26,8 +26,8 @@ ebb0(v0: i32):
     return v1
 }
 ; check: $(cst=$V) = iconst.i32 0x3b9a_ca00
-; check: $v1 = iadd $v0, $cst
-; check: return $v1
+; check: v1 = iadd v0, $cst
+; check: return v1
 
 function %bitclear(i32, i32) -> i32 {
 ebb0(v0: i32, v1: i32):