[mips] Move ABI-dependent register selections to MipsABIInfo. NFC.

Summary:
For example, a common idiom was 'isN64 ? Mips::SP_64 : Mips::SP'. This has
been moved to MipsABIInfo and replaced with 'ABI.GetStackPtr()'.

There are others that should also be moved. This patch sticks to the ones that
are obviously non-functional. The others have minor mistakes that need fixing
at the same time, mostly involving checks for 64-bit GPR's instead of checks
for 64-bit pointers.

Reviewers: tomatabacu

Reviewed By: tomatabacu

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8972

llvm-svn: 235173