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

[Simon Atanasyan]

Just curious, does it have a sense to zero- or sign- extend 32-bit types in case of O32 ABI? For example, PowerPC does the similar things in case of 64-bit ABI only `PPC64_SVR4_ABIInfo::isPromotableTypeForABI`.

http://reviews.llvm.org/D5961