Use Register over unsigned in LateEHPrepare (NFC)

Summary:
While D65962 is pending for review, I landed D65475 that added one more
use of `unsigned`. Changed it to `Register`.

Reviewers: dsanders

Subscribers: dschuff, sbc100, jgravelle-google, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66064

llvm-svn: 368727