i965/fs: Fail the shader compile instead of asserting when we can't spill

i965/fs: Fail the shader compile instead of asserting when we can't spill

Blorp doesn't handle spilling so we set allow_spilling to false in that
case.  The blorp 16x MSAA resolve shader spills in 16-wide but not 8-wide.
This commit makes it so that we fail the 16-wide compile and successfully
fall back to 8-wide instead of just assert-failing when trying to compile
the 16-wide shader.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>