[llvm] [SLP]Support revectorization of the previously vectorized scalars (PR #133091)

[via llvm-commits]

bgra8 wrote:

@alexey-bataev we have bisected an assertion in `clang` (compiled with assertions enabled `-UNDEBUG`) down to this revision. Please find attached a reduced reproducer ([repro.zip](https://github.com/user-attachments/files/19780844/repro.zip))

The compilation command:

```
$ clang -cc1 -triple x86_64-generic-linux-gnu \
    -emit-obj -relaxed-aliasing \
    -fmath-errno -target-feature +sse4.2 \
    -O3 -std=gnu++20 -vectorize-slp \
    -o /tmp/repro.o \
    -x c++-cpp-output repro.cc 
```

Output:

```
Instruction does not dominate all uses!
  %27 = load <2 x float>, ptr %arrayidx.i.i.1.i7.i, align 4, !noalias !22
  %23 = insertelement <2 x float> %27, float 0.000000e+00, i64 1
in function _ZN3SO3I2adIfLi3EEE3expERK6VectorIS1_Li3E6Linear11VectorOwnerILm1EEE
fatal error: error in backend: Broken function found, compilation aborted!
```

Can you please take a look?


https://github.com/llvm/llvm-project/pull/133091