[LLVM][IR] Use splat syntax when printing Constant[Data]Vector. (#112548)

[InstCombine] Remove some of the complexity-based canonicalization (#91185)

The idea behind this canonicalization is that it allows us to handle less
patterns, because we know that some will be can

[InstCombine] Remove some of the complexity-based canonicalization (#91185)

The idea behind this canonicalization is that it allows us to handle less
patterns, because we know that some will be canonicalized away. This is
indeed very useful to e.g. know that constants are always on the right.

However, this is only useful if the canonicalization is actually
reliable. This is the case for constants, but not for arguments: Moving
these to the right makes it look like the "more complex" expression is
guaranteed to be on the left, but this is not actually the case in
practice. It fails as soon as you replace the argument with another
instruction.

The end result is that it looks like things correctly work in tests,
while they actually don't. We use the "thwart complexity-based
canonicalization" trick to handle this in tests, but it's often a
challenge for new contributors to get this right, and based on the
regressions this PR originally exposed, we clearly don't get this right
in many cases.

For this reason, I think that it's better to remove this complexity
canonicalization. It will make it much easier to write tests for
commuted cases and make sure that they are handled.

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[PatternMatch] Do not accept undef elements in m_AllOnes() and friends (#88217)

Change all the cstval_pred_ty based PatternMatch helpers (things like
m_AllOnes and m_Zero) to only allow poison elem

[PatternMatch] Do not accept undef elements in m_AllOnes() and friends (#88217)

Change all the cstval_pred_ty based PatternMatch helpers (things like
m_AllOnes and m_Zero) to only allow poison elements inside vector
splats, not undef elements.

Historically, we used to represent non-demanded elements in vectors
using undef. Nowadays, we use poison instead. As such, I believe that
support for undef in vector splats is no longer useful.

At the same time, while poison splat elements are pretty much always
safe to ignore, this is not generally the case for undef elements. We
have existing miscompiles in our tests due to this (see the
masked-merge-*.ll tests changed here) and it's easy to miss such cases
in the future, now that we write tests using poison instead of undef
elements.

I think overall, keeping support for undef elements no longer makes
sense, and we should drop it. Once this is done consistently, I think we
may also consider allowing poison in m_APInt by default, as doing that
change is much less risky than doing the same with undef.

This change involves a substantial amount of test changes. For most
tests, I've just replaced undef with poison, as I don't think there is
value in retaining both. For some tests (where the distinction between
undef and poison is important), I've duplicated tests.

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[InstCombine] Canonicalize ((X & -X) - 1) --> ((X - 1) & ~X) (PR51784)

Enables the ctpop((x & -x ) - 1) -> cttz(x, false) fold

Alive2: https://alive2.llvm.org/ce/z/EDk4h7 (((X & -X) - 1) --> (~X &

[InstCombine] Canonicalize ((X & -X) - 1) --> ((X - 1) & ~X) (PR51784)

Enables the ctpop((x & -x ) - 1) -> cttz(x, false) fold

Alive2: https://alive2.llvm.org/ce/z/EDk4h7 (((X & -X) - 1) --> (~X & (X - 1)) )

Alive2: https://alive2.llvm.org/ce/z/8Yr3XG (CTPOP -> CTTZ)

Fixes #51126

Differential Revision: https://reviews.llvm.org/D110488

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Revert rGc360955c4804e9b25017372cb4c6be7adcb216ce "[InstCombine] Canonicalize ((X & -X) - 1) --> (~X & (X - 1)) (PR51784)"

The test changes are failing on some buildbots (but not others.....).

[InstCombine] Canonicalize ((X & -X) - 1) --> (~X & (X - 1)) (PR51784)

Enables the ctpop((x & -x ) - 1) -> cttz(x, false) fold

Alive2: https://alive2.llvm.org/ce/z/EDk4h7 (((X & -X) - 1) --> (~X &

[InstCombine] Canonicalize ((X & -X) - 1) --> (~X & (X - 1)) (PR51784)

Enables the ctpop((x & -x ) - 1) -> cttz(x, false) fold

Alive2: https://alive2.llvm.org/ce/z/EDk4h7 (((X & -X) - 1) --> (~X & (X - 1)) )

Alive2: https://alive2.llvm.org/ce/z/8Yr3XG (CTPOP -> CTTZ)

Fixes #51126

Differential Revision: https://reviews.llvm.org/D110488

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[InstCombine] Prevent complexity commutation in dec_mask_commute_neg_i32

Noticed by @spatel in D110488

[InstCombine] Add tests for ((X & -X) - 1) --> (~X & (X - 1)) canonicalization

As originally suggested on D110488