gallivm: Fix assignment of unsigned values to OUT register.

gallivm: Fix assignment of unsigned values to OUT register.

TEMP is not the only register file that accept unsigned. OUT too.

Actually, what determines the appropriate type of the destination value is
not the opcode, but rather the register.

Also cleanup/simplify code.  Add a few more asserts, but also make
code more robust by handling graceful if assert fails.

This fixes segfault / assertion in the included vert-uadd.sh graw shader.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>