[Statepoint] Improve two asserts, fix some style (NFC)

Summary:
It's important that our users immediately know what gc.safepoint_poll
is. Also fix the style of the declaration of CreateGCStatepoint, in
preparation for another change that will wrap it.

Reviewers: reames

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7517

llvm-svn: 228626