[LangRef] Add integer min/max/abs intrinsics

Add LangRef specification for the llvm.abs, llvm.umin, llvm.umax,
llvm.smin, and llvm.smax integer intrinsics.

Link to RFC:
https://lists.llvm.org/pipermail/llvm-dev/2020-June/142257.html

Proposed alive2 implementation:
https://github.com/AliveToolkit/alive2/pull/353

Differential Revision: https://reviews.llvm.org/D81829