This CR collects some refactorings that were pointed out either publicly or internally that should be done after JDK-8342382 as they unduly cause code churn (renamings) in many components

Renamings:

generate_post_barrier_fast_path -> generate_post_barrier in the code generation backends

g1_write_barrier_post_c1 -> g1_post_write_barrier_c1; moved because there are existing untouched names that use the same order of terms for e.g. c2 and interpreter.