gallivm: fix out-of-bounds access with mirror_clamp_to_edge address mode

gallivm: fix out-of-bounds access with mirror_clamp_to_edge address mode

Surprising this bug survived so long, we were missing a clamp (in the
linear filtering version).
(Valgrind complained a lot about invalid reads with piglit texwrap,
I've also seen spurios failures in this test which might have
happened due to this. Valgrind probably didn't complain before the
alignment reduction in llvmpipe to 4x4 since the test is using tiny
textures so the reads were still always well within allocated area.)
While here, also do an effective clamp (after half subtraction)
of [0,length-0.5] instead of [0, length-1] which saves an instruction
(the filtering weight could be different due to this, but only if
both texels point to the same max texel so it doesn't matter).
(Both changes are borrowed from PIPE_TEX_CLAMP_TO_EDGE case.)

Note: This is a candidate for the stable branches.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>