[llvm] [IR] Drop hash map for ValueHandleBase (PR #198187)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> I accidentally found that the DenseMap lookup of ValueHandleBase takes up about ~2.7% of the compilation time: [dtcxzyw.github.io/llvm-opt-benchmark-nightly](https://dtcxzyw.github.io/llvm-opt-benchmark-nightly/)

After recent DenseMap improvements, the percentage should go down, but ValueHandle remains a weird special user of DenseMap.

https://github.com/llvm/llvm-project/pull/198187