gallivm: fix tex offsets with mirror repeat linear
gallivm: fix tex offsets with mirror repeat linear

Can't see why anyone would ever want to use this, but it was clearly broken.
This fixes the piglit texwrap offset test using this combination.

Reviewed-by: Dave Airlie <f2295d84e358395675bc8031be58672073ae065e@redhat.com>
Reviewed-by: Jose Fonseca <61b5a29db2c0650f7c8510f93eece059c1b09320@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler