Enforce single-operand form for llvm.loop.vectorize metadata (#210932)

This patch implements Phase 2 of the RFC "Enforce Single-Operand Format
for All .enable Metadata Nodes". Please refer to RFC:

https://discourse.llvm.org/t/rfc-enforce-single-operand-format-for-all-enable-metadata-nodes/90571/

The two-operand boolean form !{!"llvm.loop.vectorize.enable", i1 0/1} is
replaced by a single-operand enable/disable pair:

  !{!"llvm.loop.vectorize.enable"}    ; force vectorization
  !{!"llvm.loop.vectorize.disable"}   ; suppress vectorization

The Verifier rejects the two-operand form, AutoUpgrade rewrites old
bitcode (including the legacy llvm.vectorizer.enable tag), and the
readers and producers across LLVM, Clang, MLIR and Polly are updated.

GitOrigin-RevId: 738029a68bd265cb4e65e03f63d84c027692adaa