Oleg Grenrus authored Mar 25, 2018 and Ben Gamari committed Mar 25, 2018

LLVM, like GHC, processes flags in the order that they appear.
Consequently, we need to ensure the user-provided flags appear last so
they can override flags produced by GHC. See #14821.

Test Plan: `ghc -O2 -optlo-O2 -v3 $FILE` and ensure that `opt` and `llc`
are invoked with `-O2`.

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14821

Differential Revision: https://phabricator.haskell.org/D4421