[Mips] Conditionally remove successor block

[smaksimovic]

Summary

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.

[atanasyan]

LGTM

lib/Target/Mips/MipsBranchExpansion.cpp

void MipsBranchExpansion::splitMBB(MachineBasicBlock *MBB) {
 
   // Create a new MBB. Move instructions in MBB to the newly created MBB.
   MachineBasicBlock *NewMBB =
       MFp->CreateMachineBasicBlock(MBB->getBasicBlock());
 
   // Insert NewMBB and fix control flow.
   MachineBasicBlock *Tgt = getTargetMBB(*FirstBr);
   NewMBB->transferSuccessors(MBB);
+  if (Tgt != getTargetMBB(*LastBr))
     NewMBB->removeSuccessor(Tgt, true);
   MBB->addSuccessor(NewMBB);
   MBB->addSuccessor(Tgt);
   MFp->insert(std::next(MachineFunction::iterator(MBB)), NewMBB);
 
   NewMBB->splice(NewMBB->end(), MBB, LastBr.getReverse(), MBB->end());
 }
 
 // Fill MBBInfos.

test/CodeGen/Mips/micromips-mtc-mfc.ll

 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
-; RUN: llc -mtriple=mips -mcpu=mips32r2 -mattr=+micromips \
+; RUN: llc -mtriple=mips -mcpu=mips32r2 -mattr=+micromips -verify-machineinstrs \
 ; RUN:     -show-mc-encoding < %s | FileCheck --check-prefix=MM2 %s
-; RUN: llc -mtriple=mips -mcpu=mips32r6 -mattr=+micromips \
+; RUN: llc -mtriple=mips -mcpu=mips32r6 -mattr=+micromips -verify-machineinstrs \
 ; RUN:     -show-mc-encoding < %s | FileCheck --check-prefix=MM6 %s
 
 define double @foo(double %a, double %b) {
 ; MM2-LABEL: foo:
 ; MM2:       # %bb.0: # %entry
 ; MM2-NEXT:    mov.d $f0, $f12 # encoding: [0x54,0x0c,0x20,0x7b]
 ; MM2-NEXT:    mtc1 $zero, $f2 # encoding: [0x54,0x02,0x28,0x3b]
 ; MM2-NEXT:    mthc1 $zero, $f2 # encoding: [0x54,0x02,0x38,0x3b]