nv50/ir: swap the least-ref'd source into src1 when both const/imm

nv50/ir: swap the least-ref'd source into src1 when both const/imm

The whole point of inlining sources is to reduce loads. We can end up in
a situation where one value is used a lot of times, and one value is
used only once per instruction. The once-per-instruction one is the one
that should get inlined, but with the previous algorithm, it was given
no preference.

This flips things around to preferring putting less-referenced values
into src1 which increases the likelihood of them being inlined.

While we're at it, adjust the heuristic to not treat 0 as an immediate,
as well as (effectively) check for situations where LIMMs can't be
loaded. All this yields improvements on nvc0:

total instructions in shared programs : 6261157 -> 6255985 (-0.08%)
total gprs used in shared programs    : 945082 -> 943417 (-0.18%)
total local used in shared programs   : 30372 -> 30288 (-0.28%)
total bytes used in shared programs   : 50089256 -> 50047880 (-0.08%)

                local        gpr       inst      bytes
    helped          21         822        3332        3332
      hurt           0         278         565         565

And more importantly avoids generating really bad code with SSBOs, where
we end up checking a lot of different values (usually immediates) against
the length.

On nv50 we get comparable results, and even improve packing (bytes went
down more than instructions):

total instructions in shared programs : 6346564 -> 6341277 (-0.08%)
total gprs used in shared programs    : 728719 -> 725131 (-0.49%)
total local used in shared programs   : 3552 -> 3552 (0.00%)
total bytes used in shared programs   : 43995688 -> 43932928 (-0.14%)

                local        gpr       inst      bytes
    helped           0        1380        3252        3774
      hurt           0         287        1710        1365

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>