[SLP] Avoid -passes=instcombine stages in SLP tests

[RKSimon]

As discussed on #144933 - we shouldn't be having multiple pass stages in SLP tests as it obscures the effect of the the vectorizer - especially as in most cases slp-vectorizer and instcombine are not sequential.

The slp tests should only run -passes=slp-vectorizer in nearly all cases and if there is a need to check for additional IR then we should consider copying / moving the test to PhaseOrdering

[llvmbot]

Hi!

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[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Simon Pilgrim (RKSimon)

As discussed on #144933 - we shouldn't be having multiple pass stages in SLP tests as it obscures the effect of the the vectorizer - especially as in most cases slp-vectorizer and instcombine are not sequential.

The slp tests should only run -passes=slp-vectorizer in nearly all cases and if there is a need to check for additional IR then we should consider copying / moving the test to PhaseOrdering

[ericxu233]

Hi, I would like to contribute to this. Can anyone assign me this issue? Thanks! @RKSimon