Revert "Reapply [InstCombine] Switch foldOpIntoPhi() to use InstSimplify"

This reverts commit e1dd2cd0 because
the original commit b20e34b3 had
a dramatic increase in the build time of RTfuzzer, which caused Fuchsia
Clang toolchain builders to timeout:
https://luci-milo.appspot.com/ui/p/fuchsia/builders/toolchain.ci/clang-linux-x64/b8801248587754572961/overview