Set rop to the square root of op rounded according to rnd.
Set rop to op1 raised to the power op2, rounded according to rnd. For
mpc_pow_d
,mpc_pow_ld
,mpc_pow_si
,mpc_pow_ui
,mpc_pow_z
andmpc_pow_fr
, the imaginary part of op2 is considered as +0.