r300/compiler: Clear loop registers in vertex shaders w/o loops

r300/compiler: Clear loop registers in vertex shaders w/o loops

The loop registers weren't being cleared, so any shader that was
executed after a shader containing loops was at risk of having a loop
randomly inserted into it.

This fixes over one hundred piglit tests, although these test
only failed during full piglit runs and would pass if
run individually.  The exact number of piglit tests that this patch
fixes will vary depending on the version of piglit and the order the
tests are run.

NOTE: This is a candidate for the stable branches.