rldimi is 64-bit instruction, due to backward compatibility, it needs to
be expanded into series of rotate and masking in 32-bit environment. In
the future, we may improve bit permutation selector and remove such
direct codegen.
rldimi is 64-bit instruction, so the corresponding builtin should not
be available in 32-bit mode. Rotate amount should be in range and
cases when mask is zero needs special handling.
This change also swaps the first and second operands of rldimi/rlwimi
to match previous behavior. For masks not ending at bit 63-SH,
rotation will be inserted before rldimi.
These builtins are already there in Clang, however current codegen may
produce suboptimal results due to their complex behavior. Implement them
as intrinsics to ensure expected instructions are emitted.
