[SLP]Improve analysis of same/alternate code ops and scheduling.

Should improve compile time for analysis and vectorization.

Metric: SLP.NumVectorInstructions

Program                                                                                       SLP.NumVectorInstructions
test-suite :: External/SPEC/CINT2017speed/623.xalancbmk_s/623.xalancbmk_s.test  6380.00                   6378.00 -0.0%
test-suite :: External/SPEC/CINT2017rate/523.xalancbmk_r/523.xalancbmk_r.test   6380.00                   6378.00 -0.0%
test-suite :: External/SPEC/CINT2006/483.xalancbmk/483.xalancbmk.test           2023.00                   2022.00 -0.0%
test-suite :: External/SPEC/CINT2006/471.omnetpp/471.omnetpp.test               148.00                    146.00 -1.4%

Generated more vector instructions.

Differential Revision: https://reviews.llvm.org/D127531