Rollup merge of #136610 - Jarcho:range_idx, r=Noratrieb

Allow `IndexSlice` to be indexed by ranges.

This comes with some annoyances as the index type can no longer inferred from indexing expressions. The biggest offender for this is `IndexVec::from_fn_n(|idx| ..., n)` where the index type won't be inferred from the call site or any index expressions inside the closure.

My main use case for this is mapping a `Place` to `Range<Idx>` for value tracking where the range represents all the values the place contains.

compiler/rustc_mir_transform/src/check_pointers.rs

@@ -71,8 +71,7 @@ pub(crate) fn check_pointers<'tcx, F>(
     // statements/blocks after. Iterating or visiting the MIR in order would require updating
     // our current location after every insertion. By iterating backwards, we dodge this issue:
     // The only Locations that an insertion changes have already been handled.
-    for block in (0..basic_blocks.len()).rev() {
-        let block = block.into();
+    for block in basic_blocks.indices().rev() {
         for statement_index in (0..basic_blocks[block].statements.len()).rev() {
             let location = Location { block, statement_index };
             let statement = &basic_blocks[block].statements[statement_index];