PowerPC: Negate m_dec values in frsqrte table

[Sintendo]

This value is used in a multiplication. The result of this multiplication is then subtracted from m_base. By negating m_dec, we are free to use an addition instead.

On x64, this saves an instruction.

[Tilka]

Looks good but keep in mind that changing the interpreter and the JITs at the same time makes the unit test meaningless.

[Sintendo]

Indeed, I can confirm my changes produce identical results for the 57 double test values.

Perhaps we should split the unit test in Interpreter/Jit64, with both validating against known expected values rather than each other?

[JosJuice]

Perhaps we should split the unit test in Interpreter/Jit64, with both validating against known expected values rather than each other?

Either that or having Interpreter validate against known values and having Jit64 validate against Interpreter makes sense to me.