[llvm][CallBrPrepare] split critical edges

If we have a CallBrInst with output that's used, we need to split
critical edges so that we have some place to insert COPYs for physregs
to virtregs.

Part 2a of
https://discourse.llvm.org/t/rfc-syncing-asm-goto-with-outputs-with-gcc/65453/8.

Test cases and logic re-purposed from D138078.

Reviewed By: efriedma, void, jyknight

Differential Revision: https://reviews.llvm.org/D139872