[InstSimplify] Fold {u,s}{min,max} x, poison -> poison

Following from the discussion in #138095 (comment), these intrinsics are poison if any of their operands are poison, and are marked as such in propagatesPoison in ValueTracking.cpp.

This will help fold away leftover vectors produced by VectorCombine when scalarizing intrinsics.

Author: lukel97

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -6549,6 +6549,10 @@ Value *llvm::simplifyBinaryIntrinsic(Intrinsic::ID IID, Type *ReturnType,
     if (match(Op0, m_ImmConstant()))
       std::swap(Op0, Op1);
 
+    // Propagate poison.
+    if (isa<PoisonValue>(Op1))
+      return Op1;
+
     // Assume undef is the limit value.
     if (Q.isUndefValue(Op1))
       return ConstantInt::get(

llvm/test/Transforms/InstSimplify/maxmin_intrinsics.ll

@@ -75,7 +75,7 @@ define i81 @smax_undef(i81 %x) {
 
 define i81 @smax_poison(i81 %x) {
 ; CHECK-LABEL: @smax_poison(
-; CHECK-NEXT:    ret i81 1208925819614629174706175
+; CHECK-NEXT:    ret i81 poison
 ;
   %r = call i81 @llvm.smax.i81(i81 poison, i81 %x)
   ret i81 %r
@@ -91,7 +91,7 @@ define i3 @smin_undef(i3 %x) {
 
 define i3 @smin_poison(i3 %x) {
 ; CHECK-LABEL: @smin_poison(
-; CHECK-NEXT:    ret i3 -4
+; CHECK-NEXT:    ret i3 poison
 ;
   %r = call i3 @llvm.smin.i3(i3 %x, i3 poison)
   ret i3 %r
@@ -107,7 +107,7 @@ define <2 x i8> @umax_undef(<2 x i8> %x) {
 
 define <2 x i8> @umax_poison(<2 x i8> %x) {
 ; CHECK-LABEL: @umax_poison(
-; CHECK-NEXT:    ret <2 x i8> splat (i8 -1)
+; CHECK-NEXT:    ret <2 x i8> poison
 ;
   %r = call <2 x i8> @llvm.umax.v2i8(<2 x i8> poison, <2 x i8> %x)
   ret <2 x i8> %r
@@ -123,7 +123,7 @@ define <2 x i8> @umin_undef(<2 x i8> %x) {
 
 define <2 x i8> @umin_poison(<2 x i8> %x) {
 ; CHECK-LABEL: @umin_poison(
-; CHECK-NEXT:    ret <2 x i8> zeroinitializer
+; CHECK-NEXT:    ret <2 x i8> poison
 ;
   %r = call <2 x i8> @llvm.umin.v2i8(<2 x i8> %x, <2 x i8> poison)
   ret <2 x i8> %r