[SimpleLoopUnswitch] Increment stats counter for unswitching switch instruction

Increment statistics counter NumSwitches at unswitchNontrivialInvariants() for
unswitching a non-trivial switch instruction. This is to fix a bug that it
increments NumBranches even for the case of switch instruction.
There is no functional change in this patch.

Differential Revision: https://reviews.llvm.org/D56408

llvm-svn: 351193