Auto merge of #86383 - shamatar:slice_len_lowering, r=bjorn3

Add MIR pass to lower call to `core::slice::len` into `Len` operand

During some larger experiment with range analysis I've found that code like `let l = slice.len()` produces different MIR then one found in bound checks. This optimization pass replaces terminators that are calls to `core::slice::len` with just a MIR operand and Goto terminator.

It uses some heuristics to remove the outer borrow that is made to call `core::slice::len`, but I assume it can be eliminated, just didn't find how.

Would like to express my gratitude to `@oli-obk` who helped me a lot on Zullip

compiler/rustc_span/src/symbol.rs

@@ -680,6 +680,7 @@ symbols! {
         lateout,
         lazy_normalization_consts,
         le,
+        len,
         let_chains,
         lhs,
         lib,
@@ -1147,6 +1148,7 @@ symbols! {
         skip,
         slice,
         slice_alloc,
+        slice_len_fn,
         slice_patterns,
         slice_u8,
         slice_u8_alloc,