[PowerPC] Fix reversed bit issue in DCMX mask for "xvtstdcdp" and "xvtstdcsp" P9 implementation

Did experiments on power 9 machine, checked the outputs for NaN & Infinity+
cases with corresponding DCMX bit set. Confirmed the DCMX mask bit for NaN and
infinity+ are reversed.

This patch fixes the issue.

Patch by Victor Huang.

Differential Revision: https://reviews.llvm.org/D59384

llvm-svn: 357494