[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