[SimplifyCFG] Hoist common code for switch multi-case destinations (#165700)

Previously, hoistCommonCodeFromSuccessors did not support hoisting
common code for multi-case destinations of `switch`.

However, if all the predecessors of a given Succ are the same (i.e.,
multi-case destination), it is safe to hoist the common code from Succ
to Pred, which is what this PR does.

See discussion
#165570 (comment).
Alive2 proof: https://alive2.llvm.org/ce/z/cYuczq
Optimization Impact:
dtcxzyw/llvm-opt-benchmark#3003

Author: Camsyn