[llvm-opt-report] Left justify unrolling counts, etc.

In the left part of the reports, we have things like U<number>; if some of
these numbers use more digits than others, we don't want a space in between the
U and the start of the number. Instead, the space should come afterward. This
way it is clear that the number goes with the U and not any other optimization
indicator that might come later on the line.

llvm-svn: 283518