[mips] Teach the delay slot filler to remove needless KILL instructions.

Summary:
Previously, the presence of KILL instructions would block valid candidates
from filling a specific delay slot. With the elimination of the KILL
instructions, in the appropriate range, we are able to fill more slots and
keep the information from future def/use analysis consistent.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: hfinkel, llvm-commits

Differential Revision: http://reviews.llvm.org/D7724

llvm-svn: 235183