[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