[llvm] [VPlan] Use VPlan operand order for VPBlendRecipes. (PR #139475)

[via llvm-commits]

================
@@ -28,7 +28,7 @@ define void @test_blend_feeding_replicated_store_1(i64 %N, ptr noalias %src, ptr
 ; CHECK-NEXT:    [[TMP7:%.*]] = select <16 x i1> [[TMP6]], <16 x i1> zeroinitializer, <16 x i1> zeroinitializer
 ; CHECK-NEXT:    [[TMP8:%.*]] = xor <16 x i1> [[TMP6]], splat (i1 true)
 ; CHECK-NEXT:    [[TMP9:%.*]] = or <16 x i1> [[TMP7]], [[TMP8]]
-; CHECK-NEXT:    [[PREDPHI:%.*]] = select <16 x i1> [[TMP7]], <16 x ptr> [[BROADCAST_SPLAT]], <16 x ptr> zeroinitializer
+; CHECK-NEXT:    [[PREDPHI:%.*]] = select <16 x i1> [[TMP6]], <16 x ptr> [[BROADCAST_SPLAT]], <16 x ptr> zeroinitializer
----------------
ayalz wrote:

Ah, ok, only some entries of PREDPHI are used/live - when !TMP6, and then they must be set to null; other entries may be set arbitrarily as in undef. Setting all to null seems a simpler solution?

Such trivial "branch on false" cases deserve folding before predicating - blend is redundant and best avoided.

https://github.com/llvm/llvm-project/pull/139475