[llvm] [SCEV] Avoid recursion in PHI handling in more cases. (PR #215910)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

For example, https://github.com/llvm/llvm-project/commit/bf5579bb857793f1bf296fadfc470d441f66a558 significantly reduces the regression in SPASS (https://llvm-compile-time-tracker.com/compare.php?from=5078983f301b8ce307d2afdc02fb16e67c7a6184&to=bf5579bb857793f1bf296fadfc470d441f66a558&stat=instructions:u).  There might be some other common constructs which we can quickly prune, like calls.

https://github.com/llvm/llvm-project/pull/215910