[llvm] PHIElimination: add target hook to control reuse. (PR #163604)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> To me, the phi node elimination assumes that two phis that have identical right-hand sides can use the same virtual temp. This is true for cpu, but not true for gpu due to gpu code's complicate divergence/convergence scheme.

This is a modeling issue 



https://github.com/llvm/llvm-project/pull/163604