[mlir][LLVM] Switch `undef` for `poison` for uninitialized values (#125629)

LLVM itself is generally moving away from using `undef` and towards
using `poison`, to the point of having a lint that caches new uses of
`undef` in tests.

In order to not trip the lint on new patterns and to conform to the
evolution of LLVM
- Rename valious ::undef() methods on StructBuilder subclasses to
::poison()
- Audit the uses of UndefOp in the MLIR libraries and replace almost all
of them with PoisonOp

The remaining uses of `undef` are initializing `uninitialized` memrefs,
explicit conversions to undef from SPIR-V, and a few cases in
AMDGPUToROCDL where usage like

    %v = insertelement <M x iN> undef, iN %v, i32 0
    %arg = bitcast <M x iN> %v to i(M * N)

is used to handle "i32" arguments that are are really packed vectors of
smaller types that won't always be fully initialized.

0 files changed, 0 insertions, 0 deletions