[CGP] Remove ModifiedDT from the makeBitReverse loop

[CGP] Remove ModifiedDT from the makeBitReverse loop

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.

Differential Revision: https://reviews.llvm.org/D66366

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369283 91177308-0d34-0410-b5e6-96231b3b80d8