[llvm] [InstCombine] Set `samesign` when converting signed predicates into unsigned (PR #112642)

[Yingwei Zheng via llvm-commits]

================
@@ -1,5 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt < %s -passes=instcombine -S | FileCheck %s
+; RUN: opt < %s -passes="instcombine<no-verify-fixpoint>" -S | FileCheck %s
----------------
dtcxzyw wrote:

In function `trunc_in_loop_exit_block`:
```

define i8 @trunc_in_loop_exit_block() {
; CHECK-LABEL: @trunc_in_loop_exit_block(
; CHECK-NEXT:  entry:
; CHECK-NEXT:    br label [[LOOP:%.*]]
; CHECK:       loop:
; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP_LATCH:%.*]] ]
; CHECK-NEXT:    [[PHI:%.*]] = phi i32 [ 1, [[ENTRY]] ], [ [[IV_NEXT]], [[LOOP_LATCH]] ]
; CHECK-NEXT:    [[CMP:%.*]] = icmp samesign ult i32 [[IV]], 100
; CHECK-NEXT:    br i1 [[CMP]], label [[LOOP_LATCH]], label [[EXIT:%.*]]
; CHECK:       loop.latch:
; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i32 [[IV]], 1
; CHECK-NEXT:    br label [[LOOP]]
; CHECK:       exit:
; CHECK-NEXT:    [[TRUNC:%.*]] = trunc i32 [[PHI]] to i8
; CHECK-NEXT:    ret i8 [[TRUNC]]
;
entry:
  br label %loop

loop:
  %iv = phi i32 [ 0, %entry ], [ %iv.next, %loop.latch ]
  %phi = phi i32 [ 1, %entry ], [ %iv.next, %loop.latch ]
  %cmp = icmp ult i32 %iv, 100
  br i1 %cmp, label %loop.latch, label %exit

loop.latch:
  %iv.next = add i32 %iv, 1
  br label %loop

exit:
  %trunc = trunc i32 %phi to i8
  ret i8 %trunc
}
```
`%iv u< 100` -> infer `nsw/nuw` for `%iv.next = add i32 %iv, 1`
-> `%iv` is non-negative -> infer `samesign` for `%cmp = icmp ult i32 %iv, 100`.



https://github.com/llvm/llvm-project/pull/112642