i965: Collect GLSL src/dst regs up in generic code.

i965: Collect GLSL src/dst regs up in generic code.

This matches brw_wm_emit.c, which we'll be using shortly.  There's a
possible penalty here in that we'll allocate registers for unused channels,
since we aren't doing ref tracking like brw_wm_pass*.c does.  However, my
measurements on GM965 don't show any for either OA or UT2004 with the GLSL
path forced.