[SeparateConstOffsetFromGEP] Add test case with trunc nuw/nsw showing miscompile

Pre commit a test case for issue #154116. When redistributing
trunc over add/sub/or we may need to drop poison generating flags
from the trunc.

Author: bjope

llvm/test/Transforms/SeparateConstOffsetFromGEP/AMDGPU/rebuild-trunc.ll

@@ -0,0 +1,36 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt < %s -mtriple=amdgcn-amd-amdhsa -mcpu=gfx1200 -passes=separate-const-offset-from-gep -S | FileCheck %s
+
+; FIXME: (add (trunc nuw A), (trunc nuw B)) is more poisonous than
+;        (trunc nuw (add A, B))), so we should for example drop the
+;        nuw on the trunc when doing the rewrite here.
+define ptr @pr154116_nuw(ptr %p, i128 %i) {
+; CHECK-LABEL: define ptr @pr154116_nuw(
+; CHECK-SAME: ptr [[P:%.*]], i128 [[I:%.*]]) #[[ATTR0:[0-9]+]] {
+; CHECK-NEXT:    [[TMP1:%.*]] = trunc nuw i128 [[I]] to i64
+; CHECK-NEXT:    [[TMP2:%.*]] = getelementptr i32, ptr [[P]], i64 [[TMP1]]
+; CHECK-NEXT:    [[ARRAYIDX2:%.*]] = getelementptr i8, ptr [[TMP2]], i64 80
+; CHECK-NEXT:    ret ptr [[ARRAYIDX2]]
+;
+  %idx = add i128 %i, 20
+  %idx.conv = trunc nuw i128 %idx to i64
+  %arrayidx = getelementptr i32, ptr %p, i64 %idx.conv
+  ret ptr %arrayidx
+}
+
+; FIXME: (add (trunc nsw A), (trunc nsw B)) is more poisonous than
+;        (trunc nsw (add A, B))), so we should for example drop the
+;        nsw on the trunc when doing the rewrite here.
+define ptr @pr154116_nsw(ptr %p, i128 %i) {
+; CHECK-LABEL: define ptr @pr154116_nsw(
+; CHECK-SAME: ptr [[P:%.*]], i128 [[I:%.*]]) #[[ATTR0]] {
+; CHECK-NEXT:    [[TMP1:%.*]] = trunc nsw i128 [[I]] to i64
+; CHECK-NEXT:    [[TMP2:%.*]] = getelementptr i32, ptr [[P]], i64 [[TMP1]]
+; CHECK-NEXT:    [[ARRAYIDX2:%.*]] = getelementptr i8, ptr [[TMP2]], i64 4
+; CHECK-NEXT:    ret ptr [[ARRAYIDX2]]
+;
+  %idx = add i128 %i, 1
+  %idx.conv = trunc nsw i128 %idx to i64
+  %arrayidx = getelementptr i32, ptr %p, i64 %idx.conv
+  ret ptr %arrayidx
+}