[UpdateTestChecks] Split core functionality of add_ir_checks into add_checks

Cherry picked from D45272, also added some setup for add_asm_checks to use add_checks as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@329266 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed