Auto merge of #39456 - nagisa:mir-switchint-everywhere, r=nikomatsakis

[MIR] SwitchInt Everywhere

Something I've been meaning to do for a very long while. This PR essentially gets rid of 3 kinds of conditional branching and only keeps the most general one - `SwitchInt`. Primary benefits are such that dealing with MIR now does not involve dealing with 3 different ways to do conditional control flow. On the other hand, constructing a `SwitchInt` currently requires more code than what previously was necessary to build an equivalent `If` terminator. Something trivially "fixable" with some constructor methods somewhere (MIR needs stuff like that badly in general).

Some timings (tl;dr: slightly faster^1 (unexpected), but also uses slightly more memory at peak (expected)):

^1: Not sure if the speed benefits are because of LLVM liking the generated code better or the compiler itself getting compiled better. Either way, its a net benefit. The CORE and SYNTAX timings done for compilation without optimisation.

```
AFTER:
Building stage1 std artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
    Finished release [optimized] target(s) in 31.50 secs
    Finished release [optimized] target(s) in 31.42 secs
Building stage1 compiler artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
    Finished release [optimized] target(s) in 439.56 secs
    Finished release [optimized] target(s) in 435.15 secs

CORE: 99% (24.81 real, 0.13 kernel, 24.57 user); 358536k resident
CORE: 99% (24.56 real, 0.15 kernel, 24.36 user); 359168k resident
SYNTAX: 99% (49.98 real, 0.48 kernel, 49.42 user); 653416k resident
SYNTAX: 99% (50.07 real, 0.58 kernel, 49.43 user); 653604k resident

BEFORE:
Building stage1 std artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
    Finished release [optimized] target(s) in 31.84 secs
Building stage1 compiler artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
    Finished release [optimized] target(s) in 451.17 secs

CORE: 99% (24.66 real, 0.20 kernel, 24.38 user); 351096k resident
CORE: 99% (24.36 real, 0.17 kernel, 24.18 user); 352284k resident
SYNTAX: 99% (52.24 real, 0.56 kernel, 51.66 user); 645544k resident
SYNTAX: 99% (51.55 real, 0.48 kernel, 50.99 user); 646428k resident
```

cc @nikomatsakis @eddyb

1 files changed, 7 insertions, 3 deletions

diff --git a/src/test/codegen/match.rs b/src/test/codegen/match.rs
index ac47f6082e3..aa100da6013 100644
--- a/src/test/codegen/match.rs
+++ b/src/test/codegen/match.rs
@@ -20,9 +20,13 @@ pub enum E {
 // CHECK-LABEL: @exhaustive_match
 #[no_mangle]
 pub fn exhaustive_match(e: E) {
-// CHECK: switch{{.*}}, label %[[DEFAULT:[a-zA-Z0-9_]+]]
-// CHECK: [[DEFAULT]]:
-// CHECK-NEXT: unreachable
+// CHECK: switch{{.*}}, label %[[OTHERWISE:[a-zA-Z0-9_]+]] [
+// CHECK-NEXT: i[[TY:[0-9]+]] [[DISCR:[0-9]+]], label %[[TRUE:[a-zA-Z0-9_]+]]
+// CHECK-NEXT: ]
+// CHECK: [[TRUE]]:
+// CHECK-NEXT: br label %[[EXIT:[a-zA-Z0-9_]+]]
+// CHECK: [[OTHERWISE]]:
+// CHECK-NEXT: br label %[[EXIT:[a-zA-Z0-9_]+]]
     match e {
         E::A => (),
         E::B => (),