[SLP]Do not reduce repeated values, use scalar red ops instead.

[SLP]Do not reduce repeated values, use scalar red ops instead.

Metric: size..text

                                                     size..text                 results     results0    diff
SingleSource/Regression/C/gcc-c-torture/execute/GCC-C-execute-980605-1.test      445.00      461.00  3.6%
SingleSource/Benchmarks/Adobe-C++/loop_unroll.test                               428477.00   428445.00 -0.0%
External/SPEC/CFP2006/447.dealII/447.dealII.test                                 618849.00   618785.00 -0.0%

For all tests some extra code was optimized, GCC-C-execute has some more
inlining after

Differential Revision: https://reviews.llvm.org/D132261