Add mul to the list of ops that we canonicalize with a select to expose an identity merge
@lebedev.ri I'm not familiar with the midpoint vector tests - your comments on rGc38831e11dc33d2a83 suggest the vector tests were just for completeness, but the simplification in codegen suggests there might be additional missing DAG folds ?
@lebedev.ri This code now optimizes considerably in IR: https://llvm.godbolt.org/z/edhMY1crG
Should these test cases be updated to match?