nir/algebraic: make patterns for float conversion lowerings imprecise

nir/algebraic: make patterns for float conversion lowerings imprecise

As noted on [1], lowering patterns of the form
floatS -> floatB -> floatS ==> floatS
cannot require precision since this may cause flush denorming.

[1] 3f779013 ("nir: Add an algebraic optimization for float->double->float")

Fixes: b86305bb ("nir/algebraic: collapse conversion opcodes (many patterns)")
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Georg Lehmann <dadschoorse@gmail.com>
Signed-off-by: Isabella Basso <isabellabdoamaral@usp.br>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20965>