[FuzzMutate] Fix a bug in `connectToSink` which might invalidate the whole module.

`connectToSink` uses a value by putting it in a future instruction.
It will replace the operand of a future instruction with the current value.

However, if current value is an `Instruction` and put into a switch case, the module is invalid.
We fix that by only connecting to Br/Switch's condition, and don't touch other operands.

Will have other strategies to mutate other Br/Switch operands to be patched once this patch is passed

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D138890