[mips] Some uses of isMips64()/hasMips64() are really tests for 64-bit GPR's

Summary:
No functional change since these predicates are (currently) synonymous.

Extracted from a patch by David Chisnall
His work was sponsored by: DARPA, AFRL

Differential Revision: http://llvm-reviews.chandlerc.com/D3202

llvm-svn: 204943