[HotColdSplitting] Attach MinSize to outlined code

Outlined code is cold by assumption, so it makes sense to optimize it
for minimal code size rather than performance.

After r344869 moved the splitting pass to the end of the IR pipeline,
this does not result in much of a code size reduction. This is probably
because a comparatively small number backend transforms make use of the
MinSize hint.

Running LNT on x86_64, I see that 33/1020 binaries shrink for a total of
919 bytes of TEXT reduction. I didn't measure a significant performance
impact.

Differential Revision: https://reviews.llvm.org/D53518

llvm-svn: 345072