mesa: linear scan register allocation for shader programs

mesa: linear scan register allocation for shader programs

This is a check-point commit; not turned on yet.

Use the linear scan register allocation algorithm to re-allocate temporary
registers.  This is done by computing the live intervals for registers and
reallocating temps with that information.

For some shaders this dramatically reduces the number of temp registers
needed.

For the time being we give up on a few cases such as relative-indexed temps
and subroutine calls (but we inline most GLSL functions anyway).