Makes it more uniform for a later bulk refactoring of
"p2i | llvm-mc | llvm-objdump", so it only needs to match p2i
and not lc2i. Some of the test expectations change though.

Also there was a stray XCHECK that I don't remember why
it was XCHECK... change that to CHECK.

BUG= none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/927433002