nvc0/ir: replace cvt instructions with add to improve shader performance

nvc0/ir: replace cvt instructions with add to improve shader performance

gives me an performance boost of 0.2% in pixmark_piano on my gk106, gm204 and
gp107.

reduces the amount of generated convert instructions by roughly 30% in
shader-db.

v2: only for 32 bit operations
    move some common code out of the switch
    handle OP_SAT with modifiers
v3: only for registers and const memory
    rework if clauses
    merge isCvt into this patch
v4: merge isCvt into its use

Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>