[mips] Fix PR35071

PR35071 exposed the fact that MipsInstrInfo::removeBranch did not walk past
debug instructions when removing branches for the control flow optimizer, which
lead to duplicated conditional branches. If the target of the branch was a
removable block, only the conditional branch in the terminating position would
have it's MBB operands updated, leaving the first branch with a dangling MBB
operand. The MIPS long branch pass would then trigger an assertion when
attempting to examine the instruction with dangling MBB operand.

This resolves PR35071.

Thanks to Alex Richardson for reporting the issue!

Reviewers: atanasyan

Differential Revision: https://reviews.llvm.org/D39288

llvm-svn: 316654