[CGP] Remove ModifiedDT from the makeBitReverse loop

[craig.topper]

Summary

I don't think anything in this loop modifies the control flow and we don't restart any iteration after setting the flag.

This code was added in http://reviews.llvm.org/D16893 but looking at the test case added there the code that caused the dominator tree to change was merging blocks with their predecessor not the bitreverse optimization.

[spatel]

LGTM

llvm/lib/CodeGen/CodeGenPrepare.cpp

bool CodeGenPrepare::optimizeBlock(BasicBlock &BB, bool &ModifiedDT) {
   }
 
   bool MadeBitReverse = true;
   while (TLI && MadeBitReverse) {
     MadeBitReverse = false;
     for (auto &I : reverse(BB)) {
       if (makeBitReverse(I, *DL, *TLI)) {
         MadeBitReverse = MadeChange = true;
-        ModifiedDT = true;
         break;
       }
     }
   }
   MadeChange |= dupRetToEnableTailCallOpts(&BB, ModifiedDT);
 
   return MadeChange;
 }