Revert "gallivm: use LLVM opaque pointers in lp_bld_tgsi_soa.c"

This reverts commit 32a55651.

Fixes: #6439 (closed) #6453 (closed)

I can't readilly repro the regression except by running the CI pipeline test "i915-g33-deqp", which does not provide stack traces that I could use to pin-point the [multiple] causes. In this situation I propose to revert the full commit (which fixes i915-g33-deqp , and hopefully the r300 case as well), and later on once a clearer understanding of the bug develops to re-apply the corrected change.