Don't renumber entities in the parser.

This makes it easier to debug testcases:
 - the entity numbers in a .cton file match the entity numbers used
   within Cretonne.
 - serializing and deserializing doesn't cause indices to change.

One disadvantage is that if a .cton file uses sparse entity numbers,
deserializing to the in-memory form doesn't compact it. However, the
text format is not intended to be performance-critical, so this isn't
expected to be a big burden.

cranelift/filetests/parser/tiny.cton

@@ -58,8 +58,8 @@ ebb0(v95: i32, v96: i32, v97: b1):
     v98 = selectif.i32 eq v97, v95, v96
 }
 ; sameln: function %selectif() native {
-; nextln: ebb0(v0: i32, v1: i32, v2: b1):
-; nextln: v3 = selectif.i32 eq v2, v0, v1
+; nextln: ebb0(v95: i32, v96: i32, v97: b1):
+; nextln: v98 = selectif.i32 eq v97, v95, v96
 ; nextln: }
 
 ; Lane indexes.
@@ -136,11 +136,11 @@ ebb0:
     stack_store v2, ss2
 }
 ; sameln: function %stack() native {
-; nextln:    $ss10 = spill_slot 8
-; nextln:    $ss2 = local 4
-; nextln:    $ss3 = incoming_arg 4, offset 8
-; nextln:    $ss4 = outgoing_arg 4
-; nextln:    $ss5 = emergency_slot 4
+; check:     $ss2 = local 4
+; check:     $ss3 = incoming_arg 4, offset 8
+; check:     $ss4 = outgoing_arg 4
+; check:     $ss5 = emergency_slot 4
+; check:     $ss10 = spill_slot 8
 
 ; check: ebb0:
 ; nextln: $v1 = stack_load.i32 $ss10