[SimplifyCFG] Be even more conservative in SinkThenElseCodeToEnd

This should *actually* fix PR30244. This cranks up the workaround for PR30188 so that we never sink loads or stores of allocas.

The idea is that these should be removed by SROA/Mem2Reg, and any movement of them may well confuse SROA or just cause unwanted code churn. It's not ideal that the midend should be crippled like this, but that unwanted churn can really cause significant regressions in important workloads (tsan).

llvm-svn: 281162