Revert "indvars: Improve LFTR by eliminating truncation when comparing

against a constant."

This reverts commit r186107. It didn't handle wrapping arithmetic in the
loop correctly and thus caused the following C program to count from
0 to UINT64_MAX instead of from 0 to 255 as intended:

  #include <stdio.h>
  int main() {
    unsigned char first = 0, last = 255;
    do { printf("%d\n", first); } while (first++ != last);
  }

Full test case and instructions to reproduce with just the -indvars pass
sent to the original review thread rather than to r186107's commit.

llvm-svn: 186152