[Dominators] check indirect branches of callbr
This will be necessary to support outputs from asm goto along indirect
edges.
Test via:
$ pushd llvm/build; ninja IRTests; popd
$ ./llvm/build/unittests/IR/IRTests \
--gtest_filter=DominatorTree.CallBrDomination
Also, return nullptr in Instruction::getInsertionPointAfterDef for
CallBrInst as was recommened in
https://reviews.llvm.org/D135997#3991427. The following phab review was
folded into this commit: https://reviews.llvm.org/D140166
Link: https://discourse.llvm.org/t/rfc-syncing-asm-goto-with-outputs-with-gcc/65453/8
Reviewed By: void, efriedma, ChuanqiXu, MaskRay
Differential Revision: https://reviews.llvm.org/D135997