[Mips] Conditionally remove successor block

In MipsBranchExpansion::splitMBB, upon splitting
a block with two direct branches, remove the successor
of the newly created block (which inherits successors from
the original block) which is pointed to by the last
branch in the original block only if the targets of two
branches differ.

This is to fix the failing test when ran with
-verify-machineinstrs enabled.

Differential Revision: https://reviews.llvm.org/D53756

llvm-svn: 345821