[llvm] [DAGCombiner] Freeze maybe poison operands when folding select to logic (PR #84924)

[Björn Pettersson via llvm-commits]

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@@ -229,10 +229,41 @@ define i32 @combine_pmaddubsw_constant_sat() {
 
 ; Constant folding PMADDWD was causing an infinite loop in the PCMPGT commuting between 2 constant values.
 define i1 @pmaddwd_pcmpgt_infinite_loop() {
-; CHECK-LABEL: pmaddwd_pcmpgt_infinite_loop:
-; CHECK:       # %bb.0:
-; CHECK-NEXT:    movb $1, %al
-; CHECK-NEXT:    retq
+; SSE-LABEL: pmaddwd_pcmpgt_infinite_loop:
----------------
bjope wrote:

Btw, it is a bit weird that SelectionDAG::computeKnownBits is handling AssertZext as if the upper bits are known to be zero. At least as long as we say that AssertZext may result in a poison value that is violating the asserted condition.

But if we consider these three examples, then I think computeKnownBits on %3 should give the same result, right?
```
  %1 = AssertZext i32 %0, i1
  %2 = lshr i32 %1, 1
  %3 = freeze i32 %2
```  
```
  %1 = AssertZext i32 %0, i1
  %2 = freeze i32 %1
  %3 = lshr i32 %2, 1
``` 
```
  %1 = AssertZext i32 %0, i1
  %3 = lshr i32 %1, 1
```  


https://github.com/llvm/llvm-project/pull/84924