[InstCombine] try to reduce x86 addcarry to generic uaddo intrinsic

If we can reduce the x86-specific intrinsic to the generic op, it allows existing 
simplifications and value tracking folds. AFAICT, this always results in identical 
x86 codegen in the non-reduced case...which should be true because we semi-generically 
(too aggressively IMO) convert to llvm.uadd.with.overflow in CGP, so the DAG/isel must 
already combine/lower this intrinsic as expected.

This isn't quite what was requested in:
https://bugs.llvm.org/show_bug.cgi?id=40486
...but we want to have these kinds of folds early for efficiency and to enable greater 
simplifications. For the case in the bug report where we have:
_addcarry_u64(0, ahi, 0, &ahi)
...this gets completely simplified away in IR.

Differential Revision: https://reviews.llvm.org/D57453

llvm-svn: 352870