[llvm] [SLP]Reduce number of alternate instruction, where possible (PR #128907)

[Mikael Holmén via llvm-commits]

mikaelholmen wrote:

Hi @alexey-bataev 

The following
```
clang -cc1 -triple x86_64-unknown-linux-gnu -emit-llvm -target-cpu x86-64 -O3 -vectorize-slp -fsanitize=alignment,shift-exponent,signed-integer-overflow -fsanitize-recover=shift-exponent bbi-105049.c
```
fails with
```
Instruction does not dominate all uses!
  %43 = extractvalue { i32, i1 } %28, 0, !nosanitize !9
  %39 = insertelement <4 x i32> %38, i32 %43, i64 1
fatal error: error in backend: Broken module found, compilation aborted!
```
It starts happening with this patch, and it fails also with the latest version in 10085390c6d.

[bbi-105049.c.gz](https://github.com/user-attachments/files/19245264/bbi-105049.c.gz)



https://github.com/llvm/llvm-project/pull/128907