target/ppc: Use helper_todouble/tosingle in helper_xststdcsp

target/ppc: Use helper_todouble/tosingle in helper_xststdcsp

When computing the predicate "is this value currently formatted
for single precision", we do not want to round the value according
to the current rounding mode, nor perform a floating-point equality.
We want to see if the N bits that make up single-precision are the
only ones set within the register, and then a bitwise equality.

Fixes a bug in which a single-precision NaN is considered !SP,
because float64_eq(nan, nan) is always false.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211119160502.17432-35-richard.henderson@linaro.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>