[asan] fix debug info produced for asan-coverage=2
Don't treat 0 as a special value for int attributes.

Split the get() to not use a default value. This way
attributes can be added that have 0 as a legitimate value.
R600/SI: Try to keep i32 mul on SALU

Also fix bug this exposed where when legalizing an immediate
operand, a v_mov_b32 would be created with a VSrc dest register.