i965/fs: Add a new fs_inst::regs_written function.

i965/fs: Add a new fs_inst::regs_written function.

Certain instructions write more than one register.  Texturing, for
example, returns 4 registers.  (We set rlen to 4 even for TXS and float
shadow sampling.)  Some math functions return 2.  Most return 1.

The next commit introduces a use of this function.

NOTE: This is a candidate for the 8.0 branch (dependency of a fix).

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>