fix ice in ssa-range-prop

It used to ICE when hitting an `assert_ne!(location.block, successor.block` due
to hitting a self-dominating bb

Fixed by checking *strict* domination instead of normal one

compiler/rustc_mir_transform/src/ssa_range_prop.rs

@@ -163,8 +163,7 @@ fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Loca
                     && self.is_ssa(place) =>
             {
                 let successor = Location { block: *target, statement_index: 0 };
-                if location.dominates(successor, &self.dominators) {
-                    assert_ne!(location.block, successor.block);
+                if location.strictly_dominates(successor, &self.dominators) {
                     let val = *expected as u128;
                     let range = WrappingRange { start: val, end: val };
                     self.insert_range(place, successor, range);

tests/ui/mir/ssa-range-prop-bb-self-domination.rs

@@ -0,0 +1,22 @@
+/// Regression test for <https://github.com/rust-lang/rust/issues/155836>.
+///
+/// SsaRangeProp pass used to fail the assert when encountering self-dominating block
+/// e.g. small loops like in `a`
+
+//@ compile-flags: -Copt-level=2
+//@ build-pass
+
+use std::hint::black_box;
+fn a(d: u8) {
+    loop {
+        1 % d;
+    }
+}
+pub fn e(d: u8) {
+    if d == 0 || black_box(false) {
+        a(d);
+    }
+}
+fn main() {
+    e(1);
+}