Use FPasses in opt exactly when it is initialized.

Previously, there was a discrepancy between the population of function
passes in FPasses, and their invocation. Function passes specified on
the command line, after an optimizaton level was simply discared. This
fix PR27509.

Patch by Jesper Antonsson.

Differential Review: http://reviews.llvm.org/D20725

llvm-svn: 272770