[MC] Reduce size of MCLEBFragment (NFC)

[nikic]

This recovers more of the max-rss regression introduced by https://reviews.llvm.org/D157657.

Per http://llvm-compile-time-tracker.com/compare.php?from=dd32d26a37e22a3bce15ed8c21145b17ff5e1401&to=b4d4aba0dfbbe405f4da3494575434710f4ec9b9&stat=max-rss&linkStats=on this is an about 1.2% max-rss improvement for the LTO link stage with debuginfo.

[MaskRay]

I suggested that we can try MCEncodedFragmentWithFixups<10, 1>, e.g. <8, 0> <8, 1> <10, 1>., perhaps we can just do <8. 0>

[nikic]

I suggested that we can try MCEncodedFragmentWithFixups<10, 1>, e.g. <8, 0> <8, 1> <10, 1>., perhaps we can just do <8. 0>

Yeah, I did try these separately first. For <10, 1> to <8, 1> we get -0.7% and for <8, 1> to <8, 0> we get -0.5%. So both changes seem worthwhile.