nv50/ir: when merging immediates/consts, load directly

nv50/ir: when merging immediates/consts, load directly

When a MERGE operation gets its constraint moves added, it
susbstantially extends live ranges to be reusing an immediate from
earlier in the program (not to mention the silliness of loading an
immediate into a register, and then moving into another register).

We detect these scenarios and insert moves that take the immediate or
constbuf load directly into the register. If it's the last use, then we
can just move that operation to the closer location.

With SM35 (255 regs) we get these results:

total instructions in shared programs : 6583670 -> 6580681 (-0.05%)
total gprs used in shared programs    : 950818 -> 944261 (-0.69%)
total shared used in shared programs  : 0 -> 0 (0.00%)
total local used in shared programs   : 15328 -> 15328 (0.00%)
total bytes used in shared programs   : 60367456 -> 60339896 (-0.05%)

                local     shared        gpr       inst      bytes
    helped           0           0        4584        3186        3186
      hurt           0           0          55         968         968

I suspect they will be better for SM20 and SM30.

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