BreakCriticalEdges: do not split the critical edge from a CallBr indirect successor

Otherwise we'll fail the assertion in SplitBlockPredecessors() related
to splitting the edges from CallBr's.

Fixes: https://github.com/ClangBuiltLinux/linux/issues/1161
Fixes: https://github.com/ClangBuiltLinux/linux/issues/1252

Reviewed By: void, MaskRay, jyknight

Differential Revision: https://reviews.llvm.org/D88438