[WebAssembly] Compiler can't recognize avgr_u pattern in C

[badumbatish]

From @lukel97, currently the avgr_u is not recognized by the compiler in this godbolt https://godbolt.org/z/r6P1T6x9f

void f(unsigned char *x, unsigned char *y, int n) {
  // should have been vectorized into avgr_u instead of seperated vectorized add and logical right shift
  for (int i = 0; i < n; i++)
    x[i] = (x[i] + y[i] + 1) / 2;
}

[llvmbot]

@llvm/issue-subscribers-backend-webassembly

Author: Jasmine Tang (badumbatish)

From @lukel97, currently the `avgr_u` is not recognized by the compiler in this godbolt https://godbolt.org/z/r6P1T6x9f

void f(unsigned char *x, unsigned char *y, int n) {
  // should have been vectorized into avgr_u instead of seperated add and logical right shift
  for (int i = 0; i < n; i++)
    x[i] = (x[i] + y[i] + 1) / 2;
}

[RKSimon]

DAG should match this if WebAssemblyTargetLowering made ISD::AVGCEILU legal/custom for the appropriate vector types.