This reverts commit 76714be5fd4ace66dd9e19ce706c2e2149dd5716, fixing the
build failure that caused the revert.

The failure stemmed from the complex deinterleaving pass identifying a
series of add operations as a "complex to single reduction", so when it
tried to transform this erroneously identified pattern, it faulted. The
fix applied is to ensure that complex numbers (or patterns that match
them) are used throughout, by checking if there is a deinterleave node
amidst the graph.

This reverts commit b3eede5e1fa7ab742b86e9be22db7bccd2505b8a.

This has been breaking most AArch64 stage2 builds for 4+ hours,
reverting to get the bots back to green.

https://lab.llvm.org/buildbot/#/builders/41/builds/4172
https://lab.llvm.org/buildbot/#/builders/4/builds/4281
https://lab.llvm.org/buildbot/#/builders/199/builds/263
https://lab.llvm.org/buildbot/#/builders/198/builds/334
https://lab.llvm.org/buildbot/#/builders/143/builds/4276
https://lab.llvm.org/buildbot/#/builders/17/builds/4725

The Complex Deinterleaving pass assumes that all values emitted will
result in complex numbers, this patch aims to remove that assumption and
adds support for emitting just the real or imaginary components, not
both.

Identified with misc-include-cleaner.

This patch is moving out following intrinsics:
* vector.interleave2/deinterleave2
* vector.reverse
* vector.splice

from the experimental namespace.

All these intrinsics exist in LLVM for more than a year now, and are
widely used, so should not be considered as experimental.

It's becoming potentially unsafe to insert a PHI instruction using a plain
Instruction pointer. Switch all the remaining sites that create and insert
PHIs to use iterators instead. For example, the code in
ComplexDeinterleavingPass.cpp is definitely at-risk of mixing PHIs and
debug-info.

When replacing ComplexDeinterleavingPass::ReductionOperation, we can do it
either from the Real or Imaginary part. The correct way is to take whichever
is later in the BasicBlock, but before the patch, we just always took the
Real part.

Fixes https://github.com/llvm/llvm-project/issues/65044

Differential Revision: https://reviews.llvm.org/D159209

Cache all results of running `identifyNode`, even those that do not identify
potential complex operations. This patch prevents ComplexDeinterleaving pass
from repeatedly trying to identify Nodes for the same pair of instructions.

Fixes https://github.com/llvm/llvm-project/issues/64379

Differential Revision: https://reviews.llvm.org/D156916

AArch64 introduced CMLA and CADD instructions as part of SVE2. This
change allows to generate such instructions when this architecture
feature is available.

Differential Revision: https://reviews.llvm.org/D153808