[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204943 91177308-0d34-0410-b5e6-96231b3b80d8