llc: Create custom pass pipeline for .mir files; NFCI

Create a custom pass pipeline when loading .mir files even in
--start-after/--start-before cases.

This streamlines the mir handling code and prepares for an upcoming
commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304755 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed