[LLVM] BasicTTIImpl allow unknown type during legality checking (#89848)

Make BasicTTIImplBase's `isTypeLegal` check handle unknown types.
Current behavior is aborting.

Motivated by a use case in SimplifyCFG, where `isTypeLegal` is called on
a struct type and dies, when it could be treated as illegal and skipped.
In general it could make sense for unknown types to be allowed, and by
default just considered not legal, but the behavior can of course be
overriden.

llvm/include/llvm/CodeGen/BasicTTIImpl.h

@@ -426,7 +426,7 @@ class BasicTTIImplBase : public TargetTransformInfoImplCRTPBase<T> {
   bool useAA() const { return getST()->useAA(); }
 
   bool isTypeLegal(Type *Ty) {
-    EVT VT = getTLI()->getValueType(DL, Ty);
+    EVT VT = getTLI()->getValueType(DL, Ty, /*AllowUnknown=*/true);
     return getTLI()->isTypeLegal(VT);
   }
 

llvm/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll

@@ -2068,3 +2068,37 @@ cond.end:                                         ; preds = %entry, %cond.false
   %conv = sext i3 %cond to i8
   ret i8 %conv
 }
+
+; Don't create a table with an unknown type
+define { i8, i8 } @test_unknown_result_type(i8 %n) {
+; CHECK-LABEL: @test_unknown_result_type(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    switch i8 [[N:%.*]], label [[SW_DEFAULT:%.*]] [
+; CHECK-NEXT:      i8 0, label [[RETURN:%.*]]
+; CHECK-NEXT:      i8 1, label [[RETURN]]
+; CHECK-NEXT:      i8 2, label [[RETURN]]
+; CHECK-NEXT:    ]
+; CHECK:       sw.default:
+; CHECK-NEXT:    [[TMP0:%.*]] = insertvalue { i8, i8 } undef, i8 0, 0
+; CHECK-NEXT:    [[TMP1:%.*]] = insertvalue { i8, i8 } [[TMP0]], i8 1, 1
+; CHECK-NEXT:    br label [[RETURN]]
+; CHECK:       return:
+; CHECK-NEXT:    [[RETVAL_0:%.*]] = phi { i8, i8 } [ undef, [[ENTRY:%.*]] ], [ undef, [[ENTRY]] ], [ undef, [[ENTRY]] ], [ [[TMP1]], [[SW_DEFAULT]] ]
+; CHECK-NEXT:    ret { i8, i8 } [[RETVAL_0]]
+;
+entry:
+  switch i8 %n, label %sw.default [
+  i8 0, label %return
+  i8 1, label %return
+  i8 2, label %return
+  ]
+
+sw.default:                                       ; preds = %entry
+  %0 = insertvalue { i8, i8 } undef, i8 0, 0
+  %1 = insertvalue { i8, i8 } %0, i8 1, 1
+  br label %return
+
+return:                                           ; preds = %sw.default, %entry, %entry, %entry
+  %retval.0 = phi { i8, i8 } [ undef, %entry ], [ undef, %entry ], [ undef, %entry ], [ %1, %sw.default ]
+  ret { i8, i8 } %retval.0
+}