This patch introduces a new command-line option for clang, namely,
amdgpu-precise-mem-op. When this option is specified, a waitcnt instruction
is generated after each memory load/store instruction. The counter values are
always 0, but which counters are involved depends on the memory instruction.

for the backend.

to the test file.

tail_call_byval_align16() to cover buffer_load/store, and
scratch_load/store.

waitcnt instruction. If so, do not insert another waitcnt. Also
add a testcase that has ds_add_rtn. Formatting change made to
SIMemoryLegalizer.cpp is reverted.

…diately

after a load/store is not necessary.