[Mips] Avoid RegScavenger::forward in Mips16InstrInfo

[foad]

Summary

RegScavenger::backward is preferred because it does not rely on accurate
kill flags.

llvm/lib/Target/Mips/Mips16InstrInfo.cpp

unsigned Mips16InstrInfo::loadImmediate(unsigned FrameReg, int64_t Imm,
   // add T, Rx, T
   //
   RegScavenger rs;
   int32_t lo = Imm & 0xFFFF;
   NewImm = lo;
   int Reg =0;
   int SpReg = 0;
 
-  rs.enterBasicBlock(MBB);
-  rs.forward(II);
+  rs.enterBasicBlockEnd(MBB);
+  rs.backward(II);
   //
   // We need to know which registers can be used, in the case where there
   // are not enough free registers. We exclude all registers that
   // are used in the instruction that we are helping.
   //  // Consider all allocatable registers in the register class initially
   BitVector Candidates =
       RI.getAllocatableSet
       (*II->getParent()->getParent(), &Mips::CPU16RegsRegClass);