[mlir][llvm] Add invariant intrinsics (#75354)

This commit implements the LLVM IR invariant intrinsics in LLVM dialect.
These intrinsics can be used to mark a program regions in which the
contents of a specific memory object will not change.

The LLVM dialect implementation also implements the
PromotableOpInterface to ensure Mem2Reg & SROA are able to promote
pointers that are marked using the invariant intrinsics.