[mips] Promote all integral/enumeration types to the GPR width

Summary:
Ensure all integral/enumeration types are appropriately annotated with
signext/zeroext. In particular, i32 now has these attributes when using the
N32/N64 ABI. This paves the way for accurately representing the way the
N32/N64 ABI's promotes integer arguments to i64.

Reviewers: atanasyan

Reviewed By: atanasyan

Subscribers: cfe-commits, theraven

Differential Revision: http://reviews.llvm.org/D5961

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220563 91177308-0d34-0410-b5e6-96231b3b80d8