Make the following changes in the way Mips handles vector arguments and return

Make the following changes in the way Mips handles vector arguments and return
values:

- Return integer vectors in integer registers.
- Pass vector arguments in integer registers.
- Set an upper bound for argument alignment. The largest alignment is 8-byte
  for O32 and 16-byte for N32/64.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159676 91177308-0d34-0410-b5e6-96231b3b80d8