coverage: Allow niches in counter/expression IDs

There is unlikely to be any practical difference between a counter limit of
2^32 and a counter limit of (2^32 - 256).

Author: Zalathar

compiler/rustc_middle/src/mir/coverage.rs

@@ -28,7 +28,6 @@ rustc_index::newtype_index! {
     #[derive(HashStable)]
     #[encodable]
     #[orderable]
-    #[max = 0xFFFF_FFFF]
     #[debug_format = "CounterId({})"]
     pub struct CounterId {}
 }
@@ -46,7 +45,6 @@ rustc_index::newtype_index! {
     #[derive(HashStable)]
     #[encodable]
     #[orderable]
-    #[max = 0xFFFF_FFFF]
     #[debug_format = "ExpressionId({})"]
     pub struct ExpressionId {}
 }