This patch is part of the ongoing effort to find more SIMD optimization 
opportunities in OpenMP code.  Here we are looking for functions that 
have the "omp declare target" attribute that are also suitable 
candidates for automatic SIMD cloning.  I've made the filter quite 
conservative, but maybe it could be improved with some further analysis. 
  I added a command-line flag to disable this in case it is buggy :-P or 
leads to excessive code bloat without improving performance in some 
cases, otherwise the SIMD clones are generated in the same way and at 
the same optimization levels as the existing simdclone pass.

I had to modify the TARGET_SIMD_CLONE_COMPUTE_VECSIZE_AND_SIMDLEN hook 
to add a boolean argument to control diagnostics, since GCC shouldn't 
complain about types the target doesn't support in cases where the user 
didn't explicitly ask for clones to be created.  I tested on 
x86_64-linux-gnu-amdgcn, plain x86_64-linux-gnu, and aarch64-linux-gnu 
to get coverage of all 3 backends that implement this hook.  OK for 
mainline?

-Sandra