Reland "[mips] Fix multiprecision arithmetic."

For multiprecision arithmetic on MIPS, rather than using ISD::ADDE / ISD::ADDC,
get SelectionDAG to break down the operation into ISD::ADDs and ISD::SETCCs.

For MIPS, only the DSP ASE has a carry flag, so in the general case it is not
useful to directly support ISD::{ADDE, ADDC, SUBE, SUBC} nodes.

Also improve the generation code in such cases for targets with
TargetLoweringBase::ZeroOrOneBooleanContent by directly using the result of the
comparison node rather than using it in selects. Similarly for ISD::SUBE /
ISD::SUBC.

Address optimization breakage by moving the generation of MIPS specific integer
multiply-accumulate nodes to before legalization.

This revolves PR32713 and PR33424.

Thanks to Simonas Kazlauskas and Pirama Arumuga Nainar for reporting the issue!

Reviewers: slthakur

Differential Revision: https://reviews.llvm.org/D33494

The previous version of this patch was too aggressive in producing fused
integer multiple-addition instructions.

llvm-svn: 307906