i965/fs: Replace nested ternary with if ladder.

i965/fs: Replace nested ternary with if ladder.

Since the types of the expression were

   bool ? src_reg : (bool ? brw_reg : brw_reg)

the result of the second (nested) ternary would be implicitly
converted to a src_reg by the src_reg(struct brw_reg) constructor. I.e.,

   bool ? src_reg : src_reg(bool ? brw_reg : brw_reg)

In the next patch, I make backend_reg (the parent of src_reg) inherit
from brw_reg, which changes this expression to return brw_reg, which
throws away any fields that exist in the classes derived from brw_reg.
I.e.,

   src_reg(bool ? brw_reg(src_reg) : bool ? brw_reg : brw_reg)

Generally this code was gross, and wasn't actually shorter or easier to
read than an if ladder.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>