[mips] Return an ArrayRef from MipsCC::intArgRegs() and remove MipsCC::numIntArgRegs()

Summary: No functional change.

Reviewers: vmedic

Reviewed By: vmedic

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5265

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217485 91177308-0d34-0410-b5e6-96231b3b80d8