[update_(llc_)test_checks.py] Support pre-processing commands
This has been rather useful in our downstream CHERI target where we want
to run tests both with addrspace(0) and addrspace(200) pointers.
With this patch we can prefix the opt command with
sed -e 's/addrspace(200)/addrspace(0)/g' -e 's/-A200-P200-G200//g' to
test both cases using the same IR input.
Reviewed By: jdoerfert
Differential Revision: https://reviews.llvm.org/D95137