Loops can only be unrolled if their loop index variable is not being
modified in the loop body.

Also check that the increment step of the loop operates on the initial
index variable.

Also remove some UNIMPLEMENTED's that were benign.

Bug chromium:845103
Bug chromium:843867
Bug skia:7846

Change-Id: Ib2b39f2d58763f0299ce7f6f75a8a75e6bdc7963
Reviewed-on: https://swiftshader-review.googlesource.com/18988Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>