i965/vec4: Simplify visit(ir_expression *)'s result_src/dst setup.

i965/vec4: Simplify visit(ir_expression *)'s result_src/dst setup.

Using dst_reg(this, ir->type) automatically sets the writemask to the
proper size for the type; src_reg(dst_reg) preserves that.  This should
be equivalent, but less code.

Note that src_reg(dst_reg) either uses SWIZZLE_XXXX or SWIZZLE_XYZW, so
the old code did need the manual writemask adjustment, since it
constructed the registers the other way around.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>