broadcom/compiler: make opt passes set current block

broadcom/compiler: make opt passes set current block

Typically, optimization passes go through all the blocks in a shader
and make adjustments on the fly, so we always want them to update
the current block or the current block pointer will become outdated.

Also, we don't need to keep track of the previous current block
pointer to restore it, since optimization passes run after we have
completed conversion to VIR, and therefore, anything that comes after
that should always set the current block before emitting code.

Fixes debug assert crashes when running shader-db:
vir.c:1888: try_opt_ldunif: Assertion `found || &c->cur_block->instructions == c->cursor.link' failed

Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13625>