Clean up MIR match lowering

* Adjust fake borrows to only be live over guards.
* Remove unused `slice_len_checked` field.
* Split the methods on builder into those for matches and those for all
  kinds of pattern bindings.

Author: matthewjasper

src/librustc_mir/build/matches/mod.rs

@@ -72,7 +72,6 @@ impl<'pat, 'tcx> MatchPair<'pat, 'tcx> {
         MatchPair {
             place,
             pattern,
-            slice_len_checked: false,
         }
     }
 }

src/librustc_mir/build/matches/test.rs

@@ -41,46 +41,46 @@ fn main() {
 //         StorageLive(_3);
 //         _3 = const true;
 //         FakeRead(ForMatchedPlace, _3);
-//         switchInt(_3) -> [false: bb11, otherwise: bb10];
+//         switchInt(_3) -> [false: bb9, otherwise: bb8];
 //     }
 //     bb4: {
 //         resume;
 //     }
 //     bb5: {
-//         _2 = const 4i32;
-//         goto -> bb14;
+//         falseEdges -> [real: bb12, imaginary: bb6];
 //     }
 //     bb6: {
-//         _0 = ();
-//         goto -> bb15;
+//         falseEdges -> [real: bb14, imaginary: bb7];
 //     }
 //     bb7: {
-//         falseEdges -> [real: bb12, imaginary: bb8];
+//         unreachable;
 //     }
 //     bb8: {
-//         falseEdges -> [real: bb13, imaginary: bb9];
+//         goto -> bb6;
 //     }
 //     bb9: {
-//         unreachable;
+//         goto -> bb5;
 //     }
 //     bb10: {
-//         goto -> bb8;
+//         FakeRead(ForLet, _2);
+//         StorageDead(_3);
+//         StorageLive(_6);
+//         _6 = &_2;
+//         _5 = const std::mem::drop(move _6) -> [return: bb19, unwind: bb4];
 //     }
 //     bb11: {
-//         goto -> bb7;
+//         _2 = const 4i32;
+//         goto -> bb10;
 //     }
 //     bb12: {
-//         goto -> bb5;
+//         goto -> bb11;
 //     }
 //     bb13: {
-//         goto -> bb6;
+//         _0 = ();
+//         goto -> bb15;
 //     }
 //     bb14: {
-//         FakeRead(ForLet, _2);
-//         StorageDead(_3);
-//         StorageLive(_6);
-//         _6 = &_2;
-//         _5 = const std::mem::drop(move _6) -> [return: bb19, unwind: bb4];
+//         goto -> bb13;
 //     }
 //     bb15: {
 //         StorageDead(_3);
@@ -96,7 +96,7 @@ fn main() {
 //     }
 //     bb18: {
 //         StorageDead(_4);
-//         goto -> bb14;
+//         goto -> bb10;
 //     }
 //     bb19: {
 //         StorageDead(_6);