[AMDGPU] Extend and reorganize memory legalizer tests

* Rename some tests to try to make a convention (where all components
  are optional) of:

    <addrspace>_<syncscope>_<memory-orders>_<operation>

* Split up at a level of granularity appropriate for the different RUN
  lines (i.e. split on addrspace so GFX6 can avoid FLAT) and that makes
  running a specific test reasonable in terms of wall time taken. This
  also means when run as part of the test suite the testing is not one
  serial bottleneck.

* Auto-generate check lines with `update_llc_test_checks.py` to make
  future maintenance more tractable.

Reviewed By: rampitec, t-tye

Differential Revision: https://reviews.llvm.org/D91545