[Test] Add regression test for PR46195.

[rickyz]

Summary

21dadd774f56778ef68c1ce307205dfbdacc793a introduced a bug causing
DAGCombiner to incorrectly order a promoted load. Add a regression test
that would have caught this.

[rickyz]

Fix typo.

[rickyz]

(One more typo fix, sorry.)

[spatel]

Sorry - I didn't get the email notification for this review before I started working on the same thing.
Can you see if rG652b375 covers it? I reduced a bit more from the test that you provided in the bug report - thanks!

[rickyz]

Ah, no worries, I'll abandon this change in favor of that one then, thanks!

llvm/test/CodeGen/X86/bug46195.ll

+; RUN: llc < %s -O1 -o /dev/null -debug-only=isel 2>&1 | FileCheck %s
+;
+; Verify that DAGCombiner preserves the correct dependency chain for
+; promoted loads.
+;
+; Generated from the following example, compiled with -O1:
+;
+; struct S {
+;   int x;
+;   unsigned short y;
+; };
+;
+; extern void maybe_mutate(struct S *ptr);
+;
+; void translate(struct S *ptr) {
+;   unsigned short c = ptr->y;
+;   maybe_mutate(ptr);
+;   ptr->y = 2 | 0x200 | (c & (0x3f | 0x8000));
+; }
+
+source_filename = "repro.c"
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+%struct.S = type { i32, i16 }
+
+define dso_local void @translate(%struct.S* %ptr) local_unnamed_addr #0 {
+entry:
+  %y = getelementptr inbounds %struct.S, %struct.S* %ptr, i64 0, i32 1
+  %0 = load i16, i16* %y, align 4, !tbaa !2
+  call void @maybe_mutate(%struct.S* %ptr) #2
+  %1 = and i16 %0, -32707
+  %2 = or i16 %1, 514
+  store i16 %2, i16* %y, align 4, !tbaa !2
+  ret void
+}
+
+declare dso_local void @maybe_mutate(%struct.S*) #2
+
+!llvm.module.flags = !{!0}
+!llvm.ident = !{!1}
+
+!0 = !{i32 1, !"wchar_size", i32 4}
+!1 = !{!"clang version 11.0.0 (https://github.com/llvm/llvm-project b28167928d2722e8774da3fce8b3307a0fa27245)"}
+!2 = !{!3, !7, i64 4}
+!3 = !{!"S", !4, i64 0, !7, i64 4}
+!4 = !{!"int", !5, i64 0}
+!5 = !{!"omnipotent char", !6, i64 0}
+!6 = !{!"Simple C/C++ TBAA"}
+!7 = !{!"short", !5, i64 0}
+
+; Verify that the the load of `y` is ordered before the call to
+; `maybe_mutate()`.
+;
+; CHECK: Optimized legalized selection DAG: %bb.0 'translate:entry'
+; CHECK: SelectionDAG has {{.*}} nodes:
+; CHECK-DAG: {{.*}}callseq_start [[LOAD:t[0-9]+]]:1,
+; CHECK-DAG: [[LOAD]]: i32,ch = load<(load 2 from %ir.y,
+; CHECK: ===== Instruction selection begins: %bb.0 'entry'