[ValueTracking] Fix KnownBits conflict for poison-only vector

If all the demanded elements are poison, return unknown instead of
conflict to avoid downstream assertions.

Fixes llvm#75505.

Author: nikic

llvm/lib/Analysis/ValueTracking.cpp

@@ -1826,6 +1826,8 @@ void computeKnownBits(const Value *V, const APInt &DemandedElts,
       Known.Zero &= ~Elt;
       Known.One &= Elt;
     }
+    if (Known.hasConflict())
+      Known.resetAll();
     return;
   }
 
@@ -1849,6 +1851,8 @@ void computeKnownBits(const Value *V, const APInt &DemandedElts,
       Known.Zero &= ~Elt;
       Known.One &= Elt;
     }
+    if (Known.hasConflict())
+      Known.resetAll();
     return;
   }
 

llvm/test/Analysis/ValueTracking/pr75505.ll

@@ -0,0 +1,17 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt -S -passes=instsimplify < %s | FileCheck %s
+
+; Just make sure that we don't assert.
+define i32 @test(<2 x i16> %a, i32 %b) {
+; CHECK-LABEL: define i32 @test(
+; CHECK-SAME: <2 x i16> [[A:%.*]], i32 [[B:%.*]]) {
+; CHECK-NEXT:    [[MUL:%.*]] = mul <2 x i16> [[A]], <i16 -1, i16 poison>
+; CHECK-NEXT:    [[BC:%.*]] = bitcast <2 x i16> [[MUL]] to i32
+; CHECK-NEXT:    [[LSHR:%.*]] = lshr i32 [[B]], [[BC]]
+; CHECK-NEXT:    ret i32 [[LSHR]]
+;
+  %mul = mul <2 x i16> %a, <i16 -1, i16 poison>
+  %bc = bitcast <2 x i16> %mul to i32
+  %lshr = lshr i32 %b, %bc
+  ret i32 %lshr
+}