ir3: Make trig replacement expression exact

ir3: Make trig replacement expression exact

This prevents other optimizations from associating the inner multiply,
which can add inaccuracies that can lead to discontinuities around the
boundary of the ffract. We should use exactly the sequence that the blob
uses to avoid problems.

Since fadd + fmul cannot be combined to ffma when exact is specified, we
have to use ffma ourselves.

Fixes artifacts in PixMark Volplosion with !7458.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12912>