[llvm] [RISCV] Insert a freeze before converting select to AND/OR. (PR #84232)

[Björn Pettersson via llvm-commits]

bjope wrote:

I think we might have hit a similar problem in our testing, but involving foldBoolSelectToLogic in generic DAGCombiner.

The IR input to llc involved
```
  %x = load i16, ptr @v_473, align 1, !tbaa !11
  %cmp1 = icmp sgt i16 %x, 0
  %sub = sub nsw i16 32767, %x
  %cmp2 = icmp sgt i16 %x, %sub
  %y = select i1 %cmp1, i1 %cmp2, i1 false
```
Resulting in a DAG with:
```
     t6: i1 = setcc t4, Constant:i16<0>, setgt:ch
        t8: i16 = sub nsw Constant:i16<32767>, t4
      t9: i1 = setcc t4, t8, setgt:ch
    t11: i1 = select t6, t9, Constant:i1<0>
```
that is combined into
```
    t6: i1 = setcc t4, Constant:i16<0>, setgt:ch
        t8: i16 = sub nsw Constant:i16<32767>, t4
      t9: i1 = setcc t4, t8, setgt:ch
    t23: i1 = and t6, t9 
```
If t6 is false the result should be zero. But after folding to the and the result would become t9 instead.
But when t6 is false, it means that t4<=0. And for t4<0 then t8 is poison, so t9 could be poisonous.

https://github.com/llvm/llvm-project/pull/84232