[WinEH] Fix inttoptr+phi optimization in presence of catchswitch

getFirstInsertionPt's return value must be checked for validity before
casting it to Instruction*. Don't attempt to insert casts after a phi in
a catchswitch block.

Fixes PR45033, introduced in D37832.

Reviewed By: davidxl, hfinkel

Differential Revision: https://reviews.llvm.org/D75381