[AMDGPU] Flip the default value of maybeAtomic. NFCI. (#75220)

In practice maybeAtomic = 0 is used to prevent SIMemoryLegalizer from
interfering with instructions that are mayLoad or mayStore but lack
MachineMemOperands. These instructions should be the exception not the
rule, so this patch sets maybeAtomic = 1 by default and only overrides
it to 0 where necessary.