[llvm] [Transforms] Speed up SSAUpdater::FindExistingPHI (PR #100281)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

The result looks good :)
```
Top 5 improvements:
  cmake/decompress.c.ll 3430208157 3287311632 -4.17%
  abc/decompress.c.ll 3387129995 3246964712 -4.14%
  clamav/bzlib.c.ll 2254636004 2200730695 -2.39%
  xgboost/stats.cc.ll 11092746376 11004268815 -0.80%
  wolfssl/server.c.ll 1046223509 1043162996 -0.29%
Top 5 regressions:
  glslang/hlslParseables.cpp.ll 2423725295 2437040423 +0.55%
  glslang/Initialize.cpp.ll 15779187901 15809604464 +0.19%
  node/libnode.node_realm.ll 2759812303 2763144520 +0.12%
  linux/xhci-pci.ll 304724871 304934080 +0.07%
  wireshark/packet-ncp.c.ll 300069591 300269769 +0.07%
Overall: -0.00043414%
```

https://github.com/llvm/llvm-project/pull/100281