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

[via llvm-commits]

================
@@ -46,8 +46,8 @@ define void @pr87378_vpinstruction_or_drop_poison_generating_flags(ptr %arg, i64
 ; CHECK-NEXT:    [[TMP20:%.*]] = xor <vscale x 8 x i1> [[TMP14]], splat (i1 true)
 ; CHECK-NEXT:    [[TMP21:%.*]] = select <vscale x 8 x i1> [[TMP13]], <vscale x 8 x i1> [[TMP20]], <vscale x 8 x i1> zeroinitializer
 ; CHECK-NEXT:    [[TMP22:%.*]] = or <vscale x 8 x i1> [[TMP19]], [[TMP21]]
-; CHECK-NEXT:    [[EXT:%.*]] = extractelement <vscale x 8 x i1> [[TMP19]], i32 0
-; CHECK-NEXT:    [[PREDPHI:%.*]] = select i1 [[EXT]], i64 [[INDEX]], i64 poison
+; CHECK-NEXT:    [[TMP23:%.*]] = extractelement <vscale x 8 x i1> [[TMP21]], i32 0
----------------
ayalz wrote:

TMP21 should complement TMP19 (at least its first element/bit) to retain semantics of flipping PREDPHI's operands. I.e., TMP13 ? TMP20 : 0 should complement TMP17 ? TMP18 : 0. Gets harder to follow.

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