[SimplifyCFG] Harden up the profitability heuristic for block splitting during sinking

[SimplifyCFG] Harden up the profitability heuristic for block splitting during sinking

Exposed by PR30244, we will split a block currently if we think we can sink at least one instruction. However this isn't right - the reason we split predecessors is so that we can sink instructions that otherwise couldn't be sunk because it isn't safe to do so - stores, for example.

So, change the heuristic to only split if it thinks it can sink at least one non-speculatable instruction.

Should fix PR30244.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@281160 91177308-0d34-0410-b5e6-96231b3b80d8