target/i386: provide 3-operand versions of unary scalar helpers

target/i386: provide 3-operand versions of unary scalar helpers

Compared to Paul's implementation, the new decoder will use a different approach
to implement AVX's merging of dst with src1 on scalar operations.  Adjust the
old SSE decoder to be compatible with new-style helpers.

The affected instructions are CVTSx2Sx, ROUNDSx, RSQRTSx, SQRTSx, RCPSx.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>