[llvm] [LV][EVL]Support reversed loads/stores. (PR #88025)

[via llvm-commits]

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@@ -9383,12 +9377,17 @@ void VPWidenLoadRecipe::execute(VPTransformState &State) {
   }
 }
 
+static Value *reverseMask(IRBuilderBase &Builder, Value *Mask, Value *EVL,
+                          Value *AllTrueMask) {
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ayalz wrote:

>> Should the above reverse of the loaded values use the partial mask of these values, rather than an all-true mask?
>> 
>> Should the above reverse of the to-be-stored values use the partial mask of these values, rather than an all-true mask?
>
> Not necessary, all-true is good enough here, real mask just increases pressure on mask register with no effect

Worth a comment that the real mask is being optimized away _here_ by a cheaper AllTrueMask.

As noted, it's "Unclear if/when "masked reverse" should be used, when reversing masked operands", i.e., 
_where_ - for which reverses, if any? - all-true would not be good enough.

Would it be better to have `createReverseEVL(IRBuilderBase &Builder, Value *Operand, Value *EVL, const Twine &Name)` generate an `experimental_vp_reverse` for masks and values alike, also filling-in the optimized-out AllTrueMask?

https://github.com/llvm/llvm-project/pull/88025