Fix cost model.

Improve the cost computation and update tests.

Modify AArch64 and RISCV tests.

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Update failing cost model tests.

Reverted due to test failures, investigating further.

Remove unused included file.

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Rebase patch and address review comments.

Remove unnecessary attribute from test.

@RKSimon I've created https://reviews.llvm.org/D128178 for the test.

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Update RUN lines to fix failing builds and add new variable argument test to SLP.

Replace undef with poison.

Address review comments.

Update diff based on reviewer's comments.

Update diff based on reviewer's comments.

Remove FIXME from test.

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Update diff based on review comments.

Updating the diff for completeness because D126055. Still looking into the reviewers comments.

Address review comments.

Hi @RKSimon, I've updated the tests for the current trunk and split them off into https://reviews.llvm.org/D126055.

Resolve another merge conflict Transforms/LoopIdiom/X86/left-shift-until-bittest.ll.

Transforms/LoopIdiom/X86/left-shift-until-bittest.ll test was recently added in this commit. The test in its current form relies on LCSSA's current behaviour. I have updated the patch to remove that reliance.

In D92576#2451875, @jmorse wrote:

LGTM, although couldn't you also just swap the basictest.ll DEBUGIFY-NEXT to be a DEBUGIFY-NOT: call void @llvm.dbg.value and not add the extra test.

(Nabeel and I share a corporate overloard, we should leave this a little more time in case anyone else wants to chip in).

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Format code with clang-format LLVM style.