[llvm] [LV][EVL] Replace VPInstruction::Select with vp.merge for predicated div/rem (PR #154072)

[Luke Lau via llvm-commits]

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@@ -2160,18 +2160,20 @@ static VPRecipeBase *optimizeMaskToEVL(VPValue *HeaderMask,
         return new VPReductionEVLRecipe(*Red, EVL, NewMask);
       })
       .Case<VPInstruction>([&](VPInstruction *VPI) -> VPRecipeBase * {
-        VPValue *LHS, *RHS;
+        VPValue *Cond, *LHS, *RHS;
         // Transform select with a header mask condition
-        //   select(header_mask, LHS, RHS)
+        //   select(mask_w/_header_mask, LHS, RHS)
         // into vector predication merge.
-        //   vp.merge(all-true, LHS, RHS, EVL)
-        if (!match(VPI, m_Select(m_Specific(HeaderMask), m_VPValue(LHS),
-                                 m_VPValue(RHS))))
+        //   vp.merge(mask_w/o_header_mask, LHS, RHS, EVL)
+        if (!match(VPI,
+                   m_Select(m_VPValue(Cond), m_VPValue(LHS), m_VPValue(RHS))))
           return nullptr;
-        // Use all true as the condition because this transformation is
-        // limited to selects whose condition is a header mask.
+
+        VPValue *NewMask = GetNewMask(Cond);
+        if (!NewMask)
+          NewMask = &AllOneMask;
----------------
lukel97 wrote:

It's not correct to transform a recipe with no header mask to a VP intrinsic with EVL, e.g. this will now transform

`select <all ones>, foo, bar -> vp.select <all ones>, foo, bar, EVL`

Which don't have the same semantics.

This is something the VPWidenLoadRecipe/VPWidenStoreRecipe/VPReductionRecipe recipes do today too, but I think we really need to fix this in #155394 first. 

If we land #155394, then we rewrite the pattern as 

```c++
        if (!match(VPI,
                   m_Select(m_RemoveMask(HeaderMask, Mask), m_VPValue(LHS), m_VPValue(RHS))))
          return nullptr;
```

Which will only transform selects that use the header mask, and should be correct

https://github.com/llvm/llvm-project/pull/154072