[VPlan] Handle BranchOnTwoConds in simplifyBranchCondition.
This fixes a crash after introducing BranchOnTwoConds (524b1788,
https://github.com/llvm/llvm-project/pull/172750) when trying to
replace BranchOnTwoConds with a VPBranchOnCond, without dissolving the
region.
In that case, we need to update the appropriate condition operand.
llvm-project
0b46cf7d - [VPlan] Handle BranchOnTwoConds in simplifyBranchCondition.
