[AArch64] Materialize constants via fneg. (#80641)

This is something that is already done as a special case for copysign,
this patch extends it to be more generally applied. If we are trying to
matrialize a negative constant (notably -0.0, 0x80000000), then there
may be no movi encoding that creates the immediate, but a fneg(movi)
might.

Some of the existing patterns for RADDHN needed to be adjusted to keep
them in line with the new immediates.