[WebAssembly] Rename SIMD min/max/avgr intrinsics for consistency

Summary:
The convention for the wasm_simd128.h intrinsics is to have the
integer sign in the lane interpretation rather than as a suffix. This
PR changes the names of the integer min, max, and avgr intrinsics to
match this convention.

Reviewers: aheejin

Subscribers: dschuff, sbc100, jgravelle-google, sunfish, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77185