[WebAssembly] Add avgr_u intrinsics and require nuw in patterns
Summary:
The vector pattern `(a + b + 1) / 2` was previously selected to an
avgr_u instruction regardless of nuw flags, but this is incorrect in
the case where either addition may have an unsigned wrap. This CL
changes the existing pattern to require both adds to have nuw flags
and adds builtin functions and intrinsics for the avgr_u instructions
because the corrected pattern is not representable in C.
Reviewers: aheejin
Subscribers: dschuff, sbc100, jgravelle-google, hiraditya, sunfish, cfe-commits, llvm-commits
Tags: #clang, #llvm
Differential Revision: https://reviews.llvm.org/D71648