[mlir] Update LLVMIR Fastmath flags use of MLIR BitEnum functionality

This diff updates the LLVMIR dialect Fastmath flags attribute to use recently
added features of `BitEnum` attributes. Specifically, this diff uses the bit
enum "group" case to represent the `fast` value as an alias for a combination
of other values (`ninf`, `nnan`, ...), instead of using a separate integer
value. (This is in line with LLVM's fastmath flags representation.) This diff
also leverages the `printBitEnumPrimaryGroups` `tblgen` field for concise
enum printing.

The `BitEnum` features were developed for an upcoming diff that adds `fastmath`
support to the arithmetic dialect. This diff simply applies some of the relevant
new features to the LLVM dialect attribute.

Reviewed By: ftynse, Mogball

Differential Revision: https://reviews.llvm.org/D124720