[SimplifyCFG] Allow dropping block that only contains ephemeral values

Perform the TryToSimplifyUncondBranchFromEmptyBlock() transform if
the block is empty except for ephemeral values. The ephemeral values
will be dropped in that case.

This makes sure that assumes don't block this transforms, as reported
in https://discourse.llvm.org/t/llvm-assume-blocks-optimization/71609.

Differential Revision: https://reviews.llvm.org/D153966