[PowerPC][Altivec] Add mfvrd and mffprd extended mnemonic

mfvrd and mffprd are both alias to mfvrsd.
This patch enables correct parsing of the aliases, but we still emit a mfvrsd.

Committing on behalf of brunoalr (Bruno Rosa).

Differential Revision: https://reviews.llvm.org/D29177

llvm-svn: 297849