Revert "i965/vec4: Change vec4_visitor::emit_lrp to use MAC for gen<6"

Revert "i965/vec4: Change vec4_visitor::emit_lrp to use MAC for gen<6"

This reverts commit 2dfbbeca50b95ccdd714d9baa4411c779f6a20d9 with the
comment about MAC and implicit accumulator removed.

Why this code didn't work in all circumstances is unknown and without a
working Ironlake simulator (which uses a different AUB format) we'll
probably never know, short of a lot of experimentation, and spending a
bunch of time to try to optimize a few instructions on Ironlake is not
time well spent.

Moreover, for mix(vec4, vec4, vec4) using the accumulator introduces a
dependence between the otherwise independent per-component calculations.
Not using the accumulator, even if it means an extra instruction per
component might be preferable. We don't know, we don't have data, and
we don't have the necessary register on Ironlake for shader_time to tell
us.

Cc: "10.2" <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=77703
Acked-by: Kenneth Graunke <kenneth@whitecape.org>