[mips] Deal with empty blocks in the mips hazard scheduler

[mips] Deal with empty blocks in the mips hazard scheduler

This patch teaches the hazard scheduler how to handle empty blocks
when search for the next real instruction when dealing with forbidden
slots.

Reviewers: slthakur

Differential Revision: https://reviews.llvm.org/D31293

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@299427 91177308-0d34-0410-b5e6-96231b3b80d8