[FastISel] Fix crash for gc.relocate lowring

Lowering safepoint checks that all gc.relocaes observed in safepoint
must be lowered. However Fast-Isel is able to skip dead gc.relocate.

To resolve this issue we just ignore dead gc.relocate in the check.

Reviewers: reames
Reviewed By: reames
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D60184

llvm-svn: 357742