Merge pull request #15301 from mikedn/cast-un

Fix inconsistent handling of zero extending casts

diff --cc src/jit/codegenxarch.cpp
Simple merge

diff --cc src/jit/compiler.h
Simple merge

diff --cc src/jit/compiler.hpp
Simple merge

diff --cc src/jit/flowgraph.cpp
Simple merge

diff --cc src/jit/gentree.cpp
Simple merge

diff --cc src/jit/gentree.h
Simple merge

diff --cc src/jit/importer.cpp
Simple merge

diff --cc src/jit/lower.cpp
Simple merge

diff --cc src/jit/lowerarmarch.cpp
Simple merge

diff --cc src/jit/lowerxarch.cpp
Simple merge

diff --cc src/jit/morph.cpp
Simple merge

diff --cc src/jit/simd.cpp
Simple merge