i965/fs: Add helper functions for generating ALU ops, like in the VS.

i965/fs: Add helper functions for generating ALU ops, like in the VS.

This gives us checking of our arguments (no more passing 1 operand to
BRW_OPCODE_MUL!), at the cost of a couple of extra parens.

v2: Rebase on gen6-if fix.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> (v1)