[InstSimplify] Consider bitcast as potential cross-lane operation

The bitcast might change the number of vector lanes, in which case
it will be a cross-lane operation.

Fixes #77320.

Author: nikic

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -4313,7 +4313,7 @@ static Value *simplifyWithOpReplaced(Value *V, Value *Op, Value *RepOp,
     // For vector types, the simplification must hold per-lane, so forbid
     // potentially cross-lane operations like shufflevector.
     if (!I->getType()->isVectorTy() || isa<ShuffleVectorInst>(I) ||
-        isa<CallBase>(I))
+        isa<CallBase>(I) || isa<BitCastInst>(I))
       return nullptr;
   }
 

llvm/test/Transforms/InstSimplify/select.ll

@@ -1734,10 +1734,15 @@ define i8 @select_or_disjoint_eq(i8 %x, i8 %y) {
   ret i8 %sel
 }
 
-; FIXME: This is a miscompile.
 define <4 x i32> @select_vector_cmp_with_bitcasts(<2 x i64> %x, <4 x i32> %y) {
 ; CHECK-LABEL: @select_vector_cmp_with_bitcasts(
-; CHECK-NEXT:    ret <4 x i32> zeroinitializer
+; CHECK-NEXT:    [[X_BC:%.*]] = bitcast <2 x i64> [[X:%.*]] to <4 x i32>
+; CHECK-NEXT:    [[Y_BC:%.*]] = bitcast <4 x i32> [[Y:%.*]] to <2 x i64>
+; CHECK-NEXT:    [[SUB:%.*]] = sub <2 x i64> [[X]], [[Y_BC]]
+; CHECK-NEXT:    [[SUB_BC:%.*]] = bitcast <2 x i64> [[SUB]] to <4 x i32>
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq <4 x i32> [[Y]], [[X_BC]]
+; CHECK-NEXT:    [[SEL:%.*]] = select <4 x i1> [[CMP]], <4 x i32> [[SUB_BC]], <4 x i32> zeroinitializer
+; CHECK-NEXT:    ret <4 x i32> [[SEL]]
 ;
   %x.bc = bitcast <2 x i64> %x to <4 x i32>
   %y.bc = bitcast <4 x i32> %y to <2 x i64>