[NewGVN] Merge unconditional branches before value numbering

[davide]

Summary

We're discussing it in https://llvm.org/bugs/show_bug.cgi?id=31468
Patch on phab for easier reviews.

[davide]

fix a typo

[davide]

fix a typo

llvm/trunk/test/Transforms/NewGVN/basic.ll

-; XFAIL: *
 ; RUN: opt < %s -newgvn -S | FileCheck %s
 
 define i32 @main() {
 block1:
   %z1 = bitcast i32 0 to i32
   br label %block2
 block2:
   %z2 = bitcast i32 0 to i32
   ret i32 %z2
 }
 
 ; CHECK: define i32 @main() {
 ; CHECK-NEXT: block1:
+; CHECK-NEXT:   br label %block2
+; CHECK: block2:
 ; CHECK-NEXT:   ret i32 0
 ; CHECK-NEXT: }