Include the test file preamble comments when building a filecheck
instance for every function in the file.
This makes it possible to define common regex variables in the preamble
and use these definitions for all the functions.
When an illegal instruction is replaced with other instructions, back up
and revisit the expanded instructions. The new instructions need to have
encodings assigned too.
This also allows for expansions to contain illegal instructions that
need to be legalized themselves.
