[SROA] Bail out on PHIs in catchswitch BBs

In the process of rewriting `alloca`s and `phi`s that use them, the SROA
pass can try to insert a non-PHI instruction by calling
`getFirstInsertionPt()`, which is not possible in a catchswitch BB. This
CL makes we bail out on these cases.

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D117168