[update_(llc_)test_checks.py] Support pre-processing commands

[arichardson]

Summary

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.

[arichardson]

Add tests and also allow it in update_llc_test_checks.py

[jdoerfert]

LGTM. Maybe wait a bit if someone else wants to take a look.

Now we also need to execute RUN lines without FileCheck and we are almost there ;)

[lebedev.ri]

Is sed even allowed to be used in tests? Is it available in the required tool set?

[jrtc27]

In D95137#2515647, @lebedev.ri wrote:

Is sed even allowed to be used in tests? Is it available in the required tool set?

There are lots of uses in llvm/test/CodeGen/ for various architectures already.

[lebedev.ri]

In D95137#2515669, @jrtc27 wrote:

In D95137#2515647, @lebedev.ri wrote:

Is sed even allowed to be used in tests? Is it available in the required tool set?

There are lots of uses in llvm/test/CodeGen/ for various architectures already.

Touché.
I'm uneasy about such tests, but the UpdateTestChecks is fine i guess.