Refactor handling of signext/zeroext in ABIArgInfo

As @rjmccall suggested in D40023, we can get rid of 
ABIInfo::shouldSignExtUnsignedType (used to handle cases like the Mips calling 
convention where 32-bit integers are always sign extended regardless of the 
sign of the type) by adding a SignExt field to ABIArgInfo. In the common case, 
this new field is set automatically by ABIArgInfo::getExtend based on the sign 
of the type. For targets that want greater control, they can use 
ABIArgInfo::getSignExtend or ABIArgInfo::getZeroExtend when necessary. This 
change also cleans up logic in CGCall.cpp.

There is no functional change intended in this patch, and all tests pass 
unchanged. As noted in D40023, Mips might want to sign-extend unsigned 32-bit 
integer return types. A future patch might modify 
MipsABIInfo::classifyReturnType to use MipsABIInfo::extendType.

Differential Revision: https://reviews.llvm.org/D41999

llvm-svn: 322396