[mips] Replace AdditionalRequires<[IsGP64bit]> with GPR_64. NFC.

Summary: Also fixed one case where HasMips64 was being used instead of IsGP64bit.

Reviewers: sdardis

Subscribers: dsanders, llvm-commits, sdardis

Differential Revision: http://reviews.llvm.org/D21028

llvm-svn: 272771