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/parser/flags.cton

@@ -18,11 +18,11 @@ ebb201:
 ebb202:
     trap oob
 }
-; check: $v1 = ifcmp_imm $v0, 17
-; check: brif eq $v1, $ebb201
-; check: brif ugt $v1, $ebb202
-; check: $v3 = ifcmp $v0, $v2
-; check: $v4 = trueif eq $v3
+; check: v1 = ifcmp_imm v0, 17
+; check: brif eq v1, ebb201
+; check: brif ugt v1, ebb202
+; check: v3 = ifcmp v0, v2
+; check: v4 = trueif eq v3
 
 function %fflags(f32) {
 ebb200(v0: f32):
@@ -40,7 +40,7 @@ ebb201:
 ebb202:
     trap oob
 }
-; check: $v2 = ffcmp $v0, $v1
-; check: brff eq $v2, $ebb201
-; check: brff ord $v2, $ebb202
-; check: $v3 = trueff gt $v2
+; check: v2 = ffcmp v0, v1
+; check: brff eq v2, ebb201
+; check: brff ord v2, ebb202
+; check: v3 = trueff gt v2