[InstCombine] Fix poison propagation in round up alignment fold

We can't directly use the high bits value if it is more poisonous
due to poison elements in the masks.

This fixes the issue reported in
#88217 (comment).

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

@@ -2965,7 +2965,8 @@ foldRoundUpIntegerWithPow2Alignment(SelectInst &SI,
     return nullptr;
 
   if (!XBiasedHighBits->hasOneUse()) {
-    if (*BiasCst == *LowBitMaskCst)
+    // We can't directly return XBiasedHighBits if it is more poisonous.
+    if (*BiasCst == *LowBitMaskCst && impliesPoison(XBiasedHighBits, X))
       return XBiasedHighBits;
     return nullptr;
   }

llvm/test/Transforms/InstCombine/integer-round-up-pow2-alignment.ll

@@ -437,14 +437,17 @@ define i8 @t17_oneuse(i8 %x) {
   ret i8 %x.roundedup
 }
 
-; Bias is equal to the alignment-1 (as opposed to alignment),
-; so we can just replace %x.roundedup with %x.biased.highbits
+; Negative test: We can't replace with %x.biased.highbits because it is
+; more poisonous.
 define <2 x i4> @t18_replacement_0b0001(<2 x i4> %x) {
 ; CHECK-LABEL: @t18_replacement_0b0001(
-; CHECK-NEXT:    [[X_BIASED:%.*]] = add <2 x i4> [[X:%.*]], <i4 3, i4 3>
+; CHECK-NEXT:    [[X_LOWBITS:%.*]] = and <2 x i4> [[X:%.*]], <i4 3, i4 3>
+; CHECK-NEXT:    [[X_LOWBITS_ARE_ZERO:%.*]] = icmp eq <2 x i4> [[X_LOWBITS]], zeroinitializer
+; CHECK-NEXT:    [[X_BIASED:%.*]] = add <2 x i4> [[X]], <i4 3, i4 3>
 ; CHECK-NEXT:    [[X_BIASED_HIGHBITS:%.*]] = and <2 x i4> [[X_BIASED]], <i4 -4, i4 poison>
 ; CHECK-NEXT:    call void @use.v2i4(<2 x i4> [[X_BIASED_HIGHBITS]])
-; CHECK-NEXT:    ret <2 x i4> [[X_BIASED_HIGHBITS]]
+; CHECK-NEXT:    [[X_ROUNDEDUP:%.*]] = select <2 x i1> [[X_LOWBITS_ARE_ZERO]], <2 x i4> [[X]], <2 x i4> [[X_BIASED_HIGHBITS]]
+; CHECK-NEXT:    ret <2 x i4> [[X_ROUNDEDUP]]
 ;
   %x.lowbits = and <2 x i4> %x, <i4 3, i4 3>
   %x.lowbits.are.zero = icmp eq <2 x i4> %x.lowbits, <i4 0, i4 0>
@@ -454,12 +457,17 @@ define <2 x i4> @t18_replacement_0b0001(<2 x i4> %x) {
   %x.roundedup = select <2 x i1> %x.lowbits.are.zero, <2 x i4> %x, <2 x i4> %x.biased.highbits
   ret <2 x i4> %x.roundedup
 }
+; Negative test: We can't replace with %x.biased.highbits because it is
+; more poisonous.
 define <2 x i4> @t18_replacement_0b0010(<2 x i4> %x) {
 ; CHECK-LABEL: @t18_replacement_0b0010(
-; CHECK-NEXT:    [[X_BIASED:%.*]] = add <2 x i4> [[X:%.*]], <i4 3, i4 poison>
+; CHECK-NEXT:    [[X_LOWBITS:%.*]] = and <2 x i4> [[X:%.*]], <i4 3, i4 3>
+; CHECK-NEXT:    [[X_LOWBITS_ARE_ZERO:%.*]] = icmp eq <2 x i4> [[X_LOWBITS]], zeroinitializer
+; CHECK-NEXT:    [[X_BIASED:%.*]] = add <2 x i4> [[X]], <i4 3, i4 poison>
 ; CHECK-NEXT:    [[X_BIASED_HIGHBITS:%.*]] = and <2 x i4> [[X_BIASED]], <i4 -4, i4 -4>
 ; CHECK-NEXT:    call void @use.v2i4(<2 x i4> [[X_BIASED_HIGHBITS]])
-; CHECK-NEXT:    ret <2 x i4> [[X_BIASED_HIGHBITS]]
+; CHECK-NEXT:    [[X_ROUNDEDUP:%.*]] = select <2 x i1> [[X_LOWBITS_ARE_ZERO]], <2 x i4> [[X]], <2 x i4> [[X_BIASED_HIGHBITS]]
+; CHECK-NEXT:    ret <2 x i4> [[X_ROUNDEDUP]]
 ;
   %x.lowbits = and <2 x i4> %x, <i4 3, i4 3>
   %x.lowbits.are.zero = icmp eq <2 x i4> %x.lowbits, <i4 0, i4 0>