[flang][fir] Add invariant attribute to fir.load (#206204)
Add an optional `invariant` unit attribute to `fir.load`, modeled on the
existing `nontemporal` attribute. When set, it indicates the referenced
memory location holds the same value at all points in the program where
it is dereferenceable, so the load may be treated as invariant.
The FIR-to-LLVM LoadOpConversion forwards the attribute to the invariant
flag of `llvm.load` (which lowers to `!invariant.load`).
`!invariant.load` metadata might be useful for OpenACC/OpenMP offload,
e.g. by using MLIR AA we can mark `fir.load`s inside the GPU kernels
letting the target codegen to generate read-only data cache load
instructions.
