Loops were no longer getting unrolled because we analyzed the entire
loop, not just the body, for changes to the index variable. Also, the
logic was inverted. Also, be conservative about loops with return
statements not being unrollable.

Bug chromium:845103

Change-Id: I5957e17f7b985ae90b10053216b6945f3f64338e
Reviewed-on: https://swiftshader-review.googlesource.com/19008Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>