[llvm] [SLP] Emit reduction instead of 2 extracts + scalar op, when vectorizing operands (PR #147583)

[via llvm-commits]

ronlieb wrote:

This patch seems to be breaking 4 spec accel2023 tests   404, 456, 457, 470
0017:   minU: 0.000000e+00 maxU: 8.777816e-03
        minU: 0.000000e+00 maxU: 8.768163e-03
                                            ^
will try to create small reproducer

https://github.com/llvm/llvm-project/pull/147583