[InstSimplify] select Cond, true, false --> Cond

[spatel]

Summary

This is step 1 of damage control assuming that we need to remove several over-reaching folds for select-of-booleans because they can cause miscompiles as shown in D72396.

The scalar case seems obviously safe:
https://rise4fun.com/Alive/jSj

And I don't think there's any danger for vectors either - if the condition is poisoned, then the select must be poisoned too, so undef elements don't make any difference.

[lebedev.ri]

LG
This is okay for vectors

llvm/lib/Analysis/InstructionSimplify.cpp

if (auto *CondC = dyn_cast<Constant>(Cond)) {
     // select true, X, Y  -> X
     if (CondC->isAllOnesValue())
       return TrueVal;
     // select false, X, Y -> Y
     if (CondC->isNullValue())
       return FalseVal;
   }
 
+  // select i1 Cond, i1 true, i1 false --> i1 Cond
+  assert(Cond->getType()->isIntOrIntVectorTy(1) &&
+         "Select must have bool or bool vector condition");
+  assert(TrueVal->getType() == FalseVal->getType() &&
+         "Select must have same types for true/false ops");
+  if (Cond->getType() == TrueVal->getType() &&
+      match(TrueVal, m_One()) && match(FalseVal, m_ZeroInt()))
+    return Cond;
+
   // select ?, X, X -> X
   if (TrueVal == FalseVal)
     return TrueVal;
 
   if (isa<UndefValue>(TrueVal))   // select ?, undef, X -> X
     return FalseVal;
   if (isa<UndefValue>(FalseVal))   // select ?, X, undef -> X
     return TrueVal;

llvm/test/Transforms/InstSimplify/select.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt < %s -instsimplify -S | FileCheck %s
 
 define i1 @bool_true_or_false(i1 %cond) {
 ; CHECK-LABEL: @bool_true_or_false(
-; CHECK-NEXT:    [[S:%.*]] = select i1 [[COND:%.*]], i1 true, i1 false
-; CHECK-NEXT:    ret i1 [[S]]
+; CHECK-NEXT:    ret i1 [[COND:%.*]]
 ;
   %s = select i1 %cond, i1 true, i1 false
   ret i1 %s
 }
 
 define <2 x i1> @bool_true_or_false_vec(<2 x i1> %cond) {
 ; CHECK-LABEL: @bool_true_or_false_vec(
-; CHECK-NEXT:    [[S:%.*]] = select <2 x i1> [[COND:%.*]], <2 x i1> <i1 true, i1 true>, <2 x i1> zeroinitializer
-; CHECK-NEXT:    ret <2 x i1> [[S]]
+; CHECK-NEXT:    ret <2 x i1> [[COND:%.*]]
 ;
   %s = select <2 x i1> %cond, <2 x i1> <i1 true, i1 true>, <2 x i1> zeroinitializer
   ret <2 x i1> %s
 }
 
 define <2 x i1> @bool_true_or_false_vec_undef(<2 x i1> %cond) {
 ; CHECK-LABEL: @bool_true_or_false_vec_undef(
-; CHECK-NEXT:    [[S:%.*]] = select <2 x i1> [[COND:%.*]], <2 x i1> <i1 undef, i1 true>, <2 x i1> <i1 false, i1 undef>
-; CHECK-NEXT:    ret <2 x i1> [[S]]
+; CHECK-NEXT:    ret <2 x i1> [[COND:%.*]]
 ;
   %s = select <2 x i1> %cond, <2 x i1> <i1 undef, i1 true>, <2 x i1> <i1 false, i1 undef>
   ret <2 x i1> %s
 }
 
 define <2 x i8> @vsel_tvec(<2 x i8> %x, <2 x i8> %y) {
 ; CHECK-LABEL: @vsel_tvec(
 ; CHECK-NEXT:    ret <2 x i8> [[X:%.*]]