Separate RemoveFalseEdges from SimplifyBranches

Otherwise dataflow state will propagate along false edges and cause
things to be marked as maybe init unnecessarily. These should be
separate, since `SimplifyBranches` also makes `if true {} else {}` into
a `goto`, which means we wouldn't lint anything in the `else` block.

compiler/rustc_mir_transform/src/remove_false_edges.rs

@@ -0,0 +1,29 @@
+use rustc_middle::mir::{Body, TerminatorKind};
+use rustc_middle::ty::TyCtxt;
+
+use crate::MirPass;
+
+/// Removes `FalseEdge` and `FalseUnwind` terminators from the MIR.
+///
+/// These are only needed for borrow checking, and can be removed afterwards.
+///
+/// FIXME: This should probably have its own MIR phase.
+pub struct RemoveFalseEdges;
+
+impl<'tcx> MirPass<'tcx> for RemoveFalseEdges {
+    fn run_pass(&self, _: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        for block in body.basic_blocks_mut() {
+            let terminator = block.terminator_mut();
+            terminator.kind = match terminator.kind {
+                TerminatorKind::FalseEdge { real_target, .. } => {
+                    TerminatorKind::Goto { target: real_target }
+                }
+                TerminatorKind::FalseUnwind { real_target, .. } => {
+                    TerminatorKind::Goto { target: real_target }
+                }
+
+                _ => continue,
+            }
+        }
+    }
+}