Allow to customize max loop unroll iterations.

The default 8 is too low for some shaders, especially given the loop body
complexity of 15 in unroll_loops. Ideally need to tweak that as well, but
externally controlled iteration count allows to set it to be very large
to force all loops to unroll.
tests: add test for another vectorize pass issue (failing now)
Fix vectorize pass going wrong with dot products
(https://bugs.freedesktop.org/show_bug.cgi?id=75224)
cleanup vectorize pass; constants are handled as rvalues right above
Merge pull request #50 from zeux/master

Allow to customize max loop unroll iterations.
tests: add a semi-complex shader with long loops, for unrolling test
increase loop size limit in unrolling (we gained some more intermediate instructions)
Max iterations API